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Sometimes, cloth has to follow the body only when being squished by it. Rigging it directly to a bone would make it unnaturally follow it when the cloth is not actually being squished!

Tutorial What you need Basic Blender knowledge: Modifying meshes, and managing of blendshapes The VRM addon for Blender Steps In Blender Import your model into Blender, and, for your own sanity, properly separate the piece of cloth you want to be squished from the rest of the clothes.
d898cf09-18dd-4bb5-aa46-96bb68c9398e-image.png Rotate the bone to its humanly possible limit. In this case, any rotation of the neck bone in the X axis greater than 5° caused clipping, but I increased the rotation to something that looks like a reasonable limit for neck rotation. Take note of all the angles you work with, in this case, 5° and 22°.
6cf2dbf8-4b68-45a9-bbba-10ca167a01ba-image.png Select your mesh, and create a new blendshape. Name it something descriptive, in this case, it's Neck Forward. Modify this blendshape such that the cloth no longer clips in the set bone position.
9f6f5dc7-a3a4-4d7c-af8b-e508748a168d-image.png (Optional) In this case, when rotating the neck in the Y axis, the cloth looks very unnatural. Since it's a common movement, you can create two additional blendshapes for each side that will activate along with the first one, blending and making the cloth follow the chin a bit better. I called these blendshapes Neck Left and Neck Right.
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c35c6581-74c0-4fd4-a619-436926d2d696-image.png Now put the bones in the rest position, reset the blendshapes, and register the new blendshapes in the VRM blendshape proxy so VNyan can read them.
84aec080-2c70-4654-94ae-dee5012b3708-image.png Export the model In VNyan

We now have to setup VNyan to check the neck rotation every frame, and if it's beyond the 5° rotation on the X axis, start activating the blendshape, blending it until the 20° limit, when Neck Corrective Down should be at its maximum.
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Here's a quick breakdown of the node setup:

This part just sets up a loop for vnyan to run every frame
52f8f060-b872-40c2-9ea4-0854bcd789f4-image.png Next, it gets the value for the X rotation
600d9a7d-484d-47a9-9290-3193bbed164f-image.png And checks that it is between the 5° and 22° range. If it passes, it sets the new blendshape value.
984b0df0-03ab-46b7-b836-9c15967ec30c-image.png The value for the blendshape is determined via a linear interpolation that maps de 5°-22° range to the 0-100 range. If your range goes from a to b, you have to substract b to make it go from 0 to a-b, and then multiply it by 100/(a-b), making the range go from 0 to 100. Here, a=5 and b=22
1cc783e6-4b4c-4b1e-b40a-4c171393c950-image.png (Optional) You can use the ordered execution node to make it also check the Y axis and apply the other two blendshapes as needed. You can just define the angle limits by what angles you used in blender while setting up the blendshapes. Don't take this screenshot too seriously, I didn't properly note the angles I used so I just guessed numbers, but it gives a general idea of the process of checking another angle along with the first one.
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And so, every frame the blendshape values are updated smoothly between 0 and 100 depending on the neck rotation, squishing the cloth as needed!
b7b93bf5-ee21-4e9d-bce1-5fc7a6971c1b-image.png Notes You can do this for a bunch of bones, use the three axis of rotation, and mix and match conditions to suit your desires. It may even start to look like a Live2D setup. Be careful though, you roughly duplicate the amount of blendshapes needed by each degree of freedom! Two bones can need up to 64 blendshapes, one for each possible range of positions! This can probably be done in Warudo too! Files The node setup for a single axis No sample file or model today, this is my friend's model (Go take a look at her work if you can understand spanish!)

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When I figured out how to do this I was OVERJOYED because I was under the impression that the only way to get the eyebrows over hair effect for a 3D model was using a shader like Poiyomi in unity, but that would require me to trap the model in a somewhat restrictive format like VSFAvatar. But turns out, MToon is a lot more versatile than expected!
I used this unity tutorial as a base, but did it in Blender with the added benefit of keeping the outlines!

Tutorial

This tutorial requires you to have some knowledge about using blender, and a vrm model to work with! We will be using Kana Fuyuko's Test-Chan. This works only on VRM 0.x models, I haven't tried this on 1.0 models.

Importing your model into blender

Start by downloading and installing the blender VRM addon, this will add the .vrm option in the blender import window. Import your .vrm model into blender
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Preparing the materials

Start by identifying the eyebrow material, usually freshly exported VRM model have a longer identifier like "N00_000_00_FaceBrow_00_FACE"
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In this material, you will set the settings in the VRM Material panel under the material selection box like in this screenshot:
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Alpha mode must be set to transparent so it can interact with other transparent materials, ZWrite Mode must be disabled or it will not work, and same with Double Sided.

Now identify the material(s) associated to the strands you want the eyebrows to be seen through, you can do this by selecting the hair faces in edit mode and see what materials light up. In this case, there's only one hair material called "Hair".

You will now create a new material, give it a good name like "Hair Stencil", assign it to these strands, and check the "Enable VRM MToon Material" box to convert it into an MToon Material, and try to copy all the settings from the Hair material manually so it looks the same, save for the Alpha mode, ZWrite mode and Double Sided settings.
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I don't just duplicate the Hair material because I've encountered an outline related bug when you duplicate VRM materials, and I'm not sure if it's fixed yet.

Hair stencil and Render Queue

Now that the front strands and the eyebrows are set to transparent, the renderer will determine which one goes first using a render queue, with materials with a lower render queue value being rendered first, and materials with a higher render queue value are rendered on top of them, so we want the eyebrows to have a higher render queue value than the front hair.

The RenderQueue Offset slider you saw in the previous screenshot IS NOT the one we will be modifying, as it is meant for MToon 1.0 as far as I know. Scroll down, and check the "Show MToon 0.0 Options" checkbox, revealing the Render Queue slider:
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This slider goes from 2951 to 3000, so we will leave the Hair Stencil material at 2955 and the eyebrow material at 2960.

Keeping the outline

If you were to export the model now, this would happen:
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This is because MToon outlines don't follow the material settings, so they just render as opaque strands with inverted normals. We will fixing by converting the outlines into an actual 3D object and putting and MToon shader on it!

First, separate the front strands, and create a duplicate of them, naming it something like "Front Outline". Now if you go to the modifier tab with that mesh selected, you will see some geometry nodes modifiers. We will convert these into geometry!
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Go to the Geometry Nodes workspace, select the modifier for the hair stencil, create a copy of the outline node tree, and change the name to something descriptive like "Outline Factory".
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Now in the node tree, move the output close to the Separate Components node, and connect its mesh output to Geometry input of the Group Output node
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This hijacks the outline creation pipeline before it converts the previously created outline geometry into an instance. I am not very knowledgeable in geometry nodes, but afaik converting geometry into an instance makes the created geometry lose all vertex weights and not be able to become actual geometry if the geometry node modifier is applied, so we skip that step.

Now we apply the modifier, and we are left with a mesh of outlines!
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Now we will disable the outlines for the Hair Stencil material, select the Hair Outline mesh and create a material for it. Convert it to MToon, set the settings like in the screenshot, and assign it to the the whole mesh:
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You can copy and paste the color from the outline settings in the rest of the hair. Now we will set the Render Queue value for this material to 2954 so the hair and eyebrows render on top of it.
d0485892-27c3-451a-b10d-8642e8b24ec1-imagen.png

If you export the model now, you will see the effect work with outlines
e051946e-0a0e-4420-a1b7-10eb1d5ce406-imagen.png

You can see some issues with the lashes, so you have to change the render queue values of those materials so they render before the outline (Have a value below 1954). After doing that, this is the result
ae528fbb-dc4b-4453-8c37-4556e74c6232-imagen.png

The cost of happiness

This looks great! It has a couple of downsides though. You may have noticed that all the outlines that usually render on the front of the hair are gone (because the outlines render after the hair), and this is more or less inevitable.

Another inevitable issue occurs when you turn to the side too much:
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This happens because the order at which faces are rendered cannot change dynamically, so when there two overlapping faces with their normals facing you, the engine may render the one that's further back instead of the one in front of it, causing this weird effect. It can be mitigated by texturing with that in mind, changing the hair shape or some other fancy thing but afaik you can never get totally rid of it, not even with other kinds of shaders.

Here's a good explanation of this issue on the minute 3:30:

Conclusion

And that's how you get outlines and eyebrow transparency on a VRM model without unity or need to use other shaders. You can also do this for eyelashes, irises or anything you can think of, but it gets increasingly more cumbersome as you juggle more materials.

I really hope this was useful and don't hesitate to ask questions, after all, this is a forum!

If you want to check out the result or take a look at the .blend I did, you can download the files here.