Sanjay Narang has compiled a great set of links on performance testing SharePoint. Check them out! This list is not specific to VSTS, and includes tool-agnostic white papers, official documentation, links to performance testing results, tooling (including VSTS), etc. Ed.
The SharePoint team has published this awesome SharePoint Server 2007 Scalability and Performance whitepaper where they tested loading up SharePoint with 50 million documents. It outlines the architecture and testing methods they used to conduct the study. Note that the team used VSTS web and load tests to load test the site. Ed.