While testing I set the texture coordinates to always be 0.0f, 0.0f, and it didn't affect the result at all. Why doesn't changing the texture coordinates have any effect? I've tried using the same shader to render a simple textured square without loading it from AssImp, and the textures work.
The only way I got anything to change was to mess with the normals (switched z and y). At least all the textures didn't look as strange as they do in the screenshot. I never really got the textures to show properly though. I have aiProcess_MakeLeftHanded enabled, so the normals should be correct. I have also tried just about every parameter I can pass to the AssImp importer, without success.
I can of course provide the code or more screenshots showing the problem, if that would be of help. One good thing about all these problems is that I've gotten more understanding about texture coordinates, buffers etc. I'd really like this to just work soon though.