SwayRig: Skirt and Hair Bones With Weights for VRChat PhysBones
Does it work with VRChat avatars from Booth and with VRoid models?
Yes. VRoid and VRM rigs and the usual Unity humanoid names are recognised. VRoid keeps clothes inside the body mesh, so pick the skirt by its Material. Bones are added to your avatar's own armature. In Unity you then add a VRC Phys Bone to Skirt_Root and Hair_Root.