maybe you could make restriction that multiple passes all need the same vertex format? for speed purposes you will probably want each mesh to have its own vertex buffer (when i say mesh i mean group of triangles with the same material/shader propeties). if you are using a vertex buffer for multiple passes, you shouldn't need to touch the vertex data. in order to truly change the vertex format per pass you would have to lock/unlock the vertex buffer and rearrange everything.

btw, in your original post you had mentioned that each pass would need a different vertex shader. thats probably a bad idea. switching between programmable shaders and fixed function is quite expensive.