One of the key things we focused on in this release of our testing tools was to not only provide a good number of test types, but to also create a framework that makes it easy for third-parties to integrate their tools with ours (the rest of the VSTS teams have taken similar steps, not just the testing tools team). In the past, Borland, Compuware, AutomatedQA, and others have put in the effort to have their tools integrate with VSTS (as well as some yet-unannounced companies). This month Mercury Interactive also announced their intentions to integrate with our framework.
For more details, read Mercury's press release.
Tom ArnoldPM, VSTEST