Custom Web Performance Rules

Create a Custom Extraction Rule for a Web Performance Test
http://msdn.microsoft.com/en-us/library/ms243179.aspx

Create a Custom Validation Rule for a Web Performance Test
http://msdn.microsoft.com/en-us/library/ms182556

Create a Web Performance Test Plug-In
http://msdn.microsoft.com/en-us/library/ms243191.aspx

Custom Extraction Rule - Extract Cookie Value
http://social.msdn.microsoft.com/Forums/en-US/vstswebtest/thread/cf2bf144-4e87-414f-8aa3-7731f9bc7b73/