[aeonik]

I used to use CheatEngine on single player games that I ran through Steam, and I don't recall Steam having a problem with that at the time. Not sure if it's changed, but it was pretty reasonable.

Never used it on Multiplayer games though.

[duskwuff]

VAC is only used in multiplayer games. (And even then, only in a limited number of games - https://steamdb.info/charts/?category=8)