Back in 2003, Korby Parnell listed a couple of tools to switch source control providers. The interesting one was Sourcecode Control Switcher by Soenke Schau. It adds it self into the task tray to make switching providers really simple. I didn't try it, but it looks like it may be a good way to go
Another possibility is to create Windows shortcuts. Make a pair of batch scripts each containing one of the reg command lines that Ed Hintz posted. Then create Windows shortcuts to each batch script, perhaps on your desktop. Right click on each and choose Properties. In the Shortcut tab you'll see a Shortcut key entry. Click in it and press a key. You could assign T (Ctrl + Alt + T) to the TFS MSSCCI provider and V (Ctrl + Alt + V) to the VSS provider.
Of course, you could also have batch files that launch VS after using the reg command line to set the provider as desired.