VAC stands for Valve Anti-Cheat, and it is a system put in place by Valve to protect games from the use of any third-party modifications to a game designed to give a player an advantage over another. This includes changes to a game's core executable files and dynamic link libraries.
How does it work?
If a user connects to a VAC-Secured server from a computer with identifiable cheats installed, the VAC system will ban the user from playing on VAC-Secured servers across all of Steam.
VAC bans are not controlled by the game developer but by Valve itself, so you will have to contact their support team here.
For further information regarding VAC, please refer to the following steam article here.