All VAC bans are permanent - Valve has a zero-tolerance policy for cheating and will not lift VAC bans on request.
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 that game on VAC-Secured servers in the future.
The VAC system reliably detects cheats using their cheat signatures. Any third-party modifications 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.
While server admins may choose to ban specific players, server admins cannot VAC ban players.
Use only trusted machines to play on VAC-Secured servers - if you are not sure whether or not the machine you are using to connect to Steam may have cheats installed, do not play on VAC-Secured servers.
Use caution when installing any game modifications, such as scripts or custom skins. Only download custom content from trusted sources. Hackers sometimes maliciously disguise their cheats to cause others to get banned.
If an account that has a phone number registered to it receives a VAC or Game ban, all other accounts that used the same phone number at the time of the infraction will also receive a ban. If the other accounts do not own the game they will still receive the ban and be unable to purchase the title on that account. The phone number on an account that receives a ban will be suspended for three months and cannot be applied to another account during that time. The cooldown duration for applying a phone number after a ban will increase each time a cooldown is applied.
This feature is not enabled for all games and will be used at the discretion of the developer.
Please see the list of VAC enabled games on the Steam Store.
The Valve Anti-Cheat system is enabled for servers displaying the Secured shield logo.
Cheating in one of the following Source games or a Source mod will result in a VAC ban for all games in the list:
If you receive a VAC ban in any of the Source Engine games listed above, that VAC ban will cause your non-purchased Team Fortress 2 inventory to be removed from the account.
Cheating in one of the following Gold Source games will result in a VAC ban for all games in the list:
Excluding the games listed above, VAC bans will only apply to the game the cheat was detected in. For example, a VAC ban in Modern Warfare 2 will not affect Modern Warfare 3.
Some VAC protected games allow servers that are not protected by VAC, known as insecure servers. Users that are VAC banned are still able to play on insecure servers for the game they are banned from. Please note that not all games have insecure servers.
Your Family Sharing privileges may be revoked and your account may also be VAC banned if your library is used by others to conduct cheating or fraud. In addition, VAC-banned games cannot be shared. We recommend you only authorize Steam Accounts and computers you know to be secure. And as always, never give your password to anyone.
The best indicator that your account has been banned by VAC is to launch Steam and go to Steam > Settings > Account tab and check VAC Status.
If you receive an error message stating "STEAMID IS BANNED" (or other similar messages) when attempting to connect to a server, your account is not banned by VAC - server administrators may ban you from a particular server (we cannot prevent this - whoever runs the server can ban players for any reason they desire).
If you find that you are banned from a large number of servers but your VAC status is in good standing, you have likely been banned by a third-party ban tool, such asSteambans or Punkbuster. These third-party tools are used by a large number of servers sharing a common database of banned IDs. These tools are unrelated to Steam and we cannot assist you in resolving this issue.
You will not be banned by the VAC system unless you login to a VAC-secure server with a cheat installed on your computer.
Read full article at : https://support.steampowered.com/kb_article.php?ref=7849-RADZ-6869