Lemme help you out with that Nat =D

dx80.vtx
-> DirectX 80 Mesh Information [triangles, mesh, skin info
dx90.vtx
-> DirectX 90 Mesh Information [triangles, mesh, skin info
sw.vtx
-> Software Mesh Information [triangles, mesh, skin info
xbox.vtx
-> Xbox Compatable Mesh Information [triangles, mesh, skin info

mdl
-> Animation; Bounding Box; Hitbox; main Skeletal

phy
-> Stored collision data for the .mdl

vvd
->Bone weights; normals; vertices; tangents and textures for the mdl.

Remove the vvd; you still have an operational model, however the model will be completely invisible.

Remove any of the others, and you will get the ERROR sign.