Some servers are VAC enabled. You can tell if you use the legacy server browser. Should be a VAC Shield in front of the server names.
You won't be VAC banned if you use LUA cheats. Only if you modify the game with DLL-injections or memory manipulation, while being on VAC enabled servers.
And if you are VAC banned, you should still be able to play singleplayer, and non-VAC enabled servers just fine. I may be wrong on that though, so don't quote me on that if you get VAC banned.