Thank you for the advice, this may be just what's wrong. However, it doesn't explain why I get the UV error after I reload my scene and not right after I convert it, but I suppose that's too much to ask for :)

I don't know which version of VRay you're using, but VRay 1.5 SP2 gives you the option to convert the entire scene into VRay, which pretty much converts the textures, and possibly optimizes/precalculates the models in order to render faster or more efficiently with VRay.
You just have to right click in an empty area of your viewport and select that option from the list.
Perhaps there are other versions of VRay that have this option, but I only know about the one I use.