R – How to migrate from SourceGear Vault to TFS

File -> Source Control -> Advanced -> Change Source Control and then unbind and/or disconnect all projects and the solution.

This should remove all bindings from the solution and project files. (After this you can switch the SCC provider in Tools -> Options -> Source Control -> Plug-in Selection).

The SCC specification prescribes that all SCC providers should implement this behavior. (I only tested it for VSS, TFS and AnkhSVN)

Have a look at the TFS Migration and Syncronization toolkit. This is an open source project developed by Microsoft for people doing this type of thing. That said I usually recommend keeping the old system around for the history rather than importing as people usually have a better experience that way rather than going through the pain of import.