From Valve Developer Community
VScript is a virtual machine for scripting that acts as an abstract binding layer between the Source engine and external scripts. According to the Alien Swarm SDK, following scripting languages are, or could be supported in near future:
VScript is currently implemented in Squirrel for these games:
VScript is currently implemented in Lua for these games:
Script files are placed and executed in
scripts/vscripts/ inside the game folder.