Hi patmark and welcome to TSF,
This VAC issue will have to be taken up with Steam. If the addon you bought was from the official website then this shouldn't of caused an issue with VAC.
If you bought this addon from a site that required your steam login details then you may of been scammed and your account hijacked. The reason why im saying this is because for the PC version of MW2 addon pack on MW2 official site leads you to the Steam Store page.
Unfortunately if thats the case and VAC has banned you (The Hijacker) for cheating then any game on that steam account that connections to a VAC secure server will be blocked. (Counter-Strike, Team Fortress 2, ect).
You dont need to go as far as formatting your HDD but certainly get rid of the addon you have installed. Also check for viruses or any other applications you know you didn't install.
Getting you off the banned list is very difficult mainly for the fact theres no way to be sure if it wasn't you that cheated (Not accusing you at all). You can however try and fight for your money back on the lost games steam might agree to it, but again they rarely do without evidence.
If you haven't already you can start a help ticket with them.
https://support.steampowered.com/login.php?
This VAC issue will have to be taken up with Steam. If the addon you bought was from the official website then this shouldn't of caused an issue with VAC.
If you bought this addon from a site that required your steam login details then you may of been scammed and your account hijacked. The reason why im saying this is because for the PC version of MW2 addon pack on MW2 official site leads you to the Steam Store page.
Unfortunately if thats the case and VAC has banned you (The Hijacker) for cheating then any game on that steam account that connections to a VAC secure server will be blocked. (Counter-Strike, Team Fortress 2, ect).
You dont need to go as far as formatting your HDD but certainly get rid of the addon you have installed. Also check for viruses or any other applications you know you didn't install.
Getting you off the banned list is very difficult mainly for the fact theres no way to be sure if it wasn't you that cheated (Not accusing you at all). You can however try and fight for your money back on the lost games steam might agree to it, but again they rarely do without evidence.
If you haven't already you can start a help ticket with them.
https://support.steampowered.com/login.php?