Since you ask, the best place to get information on VisVSA and related applications about the overall modeling is Geometric Solutions. They are a .net company on the web. It is a UGS (now Siemens) product, but the guys at Geometric know the application perhaps the best of anyone.
Basically, VisVSA uses the JT neutral format of models to add 3D GD&T to part features. It then uses those features to assemble parts together (constraints). From there you setup what you want to measure among the parts. Run monte carlo simulations to get a process report, histogram, and pareto of contributors for the measurements created.