From [Valve Website][1]:

>All VAC bans are permanent - Valve has a zero-tolerance policy for cheating and will not lift >VAC bans under any circumstances.

Also,

>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.

And

> The following will <b>not</b> 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 map

So if your mod fall into second category but not first, you should be okay :)

It might also be good to know that VAC bans don't happen instantly, usually a random time between a week - 2 months you will see this ban occur. This is to help stop the cheaters from knowing which hacks actually get pass VAC Security.

  [1]: https://support.steampowered.com/kb_article.php?ref=7849-RADZ-6869