From there, I would boot into Vista and run msconfig
On the boot tab, rename one of the Win 7 installations. Doesn't matter what you call it, just so you can identify one from the other. Hell, name one 'Kenny' and the other "Mr Bill"
On restart, try the win 7 installs. If Mr Bill works and Kenny doesn't? Then Go back into the boot tab in msconfig and kill kenny.
I will try this tomorrow