The VSS plugin isn't really one that we actively support or have much (any) knowledge about. I suggest you try to get more information about your problem from the VSS developers / forum / mailing lists on sourceforge first.
Be mindful of the fact that there are two separate distributions of STS 2.7. One of them is based on Eclipse 3.6 and another on Eclipse 3.7. It is possible that the problem you are experiencing is because of an incompatibility between Eclipse 3.7 and VSS. If that's the case, you might have better luck with the E36-based distribution.
If you find that the problem is actually specific to STS (meaning it only happens with STS based on E37 but not a base eclipse E37) then we can take a look. If so, you should raise a Jira issue here:
providing enough detailed information for us to be able reproduce the problem.
Kris De Volder -- SpringSource