.vtest is a format of Source Engine test scripts. It is the same as .cfg.
They can be used to run in-game tests or to automate processes without using .cfg files.
.vtest files must be placed in ModName\testscripts directory and can be runned by launching the engine with
-testscript [filename] command line argument (with .vtest extension).
Several working examples of testscripts can be found in the leaked -4 build of Half-Life 2.