IMPORTANT:
All VAC bans are permanent - Valve has a zero-tolerance policy for cheating and will not lift VAC bans under any circumstances.
WHAT IS VAC?

The Valve Anti-Cheat system is enabled for servers displaying the Secured shield logo. 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 in the future.
While server admins may choose to ban specific players, server admins cannot VAC ban players. The VAC system is automated - contacting Steam Support to report cheaters is not necessary, nor will Steam Support act on any information provided.
The VAC system reliably detects cheats using their cheat signatures. Any third-party modification to a game designed to give one player an advantage over another is classified as a cheat or hack and will trigger a VAC ban. This includes modifications to a game's core executable files and dynamic link libraries.
The following will not trigger a VAC ban:
Using chat programs like X-Fire
Playing multi-player mods which do not modify core .EXE and .DLL files
System hardware configurations
Updated system drivers, such as video card drivers
Using custom skins, sounds or maps