>We would like to include the project files (PJX, PJT) in VSS, but this means that the integration in VFP is not very useful.

As Viv indicated, the PJM is under source control. This is a text-only version of the PJX, which developers can short. Join Source Control Project builds you a new PJX from the PJM. Check out the articles Viv recommended, as well as Ted Roche's VSS book from Hentzenwerke.