VC is a scripting language, short for VergeC, which is a C-like language created specifically for use in creating Verge games. It is one of the two languages available for making games with Verge, the other being Lua (which you can read about here).

You create VC files in any text editor you want, and when you run verge.exe (with releasemode 0 and lua 0 set in your verge.cfg), the engine will attempt to compile your code starting with a mandatory system.vc file in the same base directory as the engine file, and then any other VC files you #include therein.