Just wanna let you guys know, there is a dvar called "seta sv_vac "0" in singleplayer for VAC 2. - Which means Vac2 is activated in singleplayer.
A bunch of people on other forums have confirmed that it is actively scanning single player executables so I think using the trainer is a pretty big risk right now.
A suggestion would be to wait about a week or two and see if any Cheathappens members get VAC banned or not to determine Valve's policy on singleplayer cheating. (We don't want a repeat of Starcraft 2 :/)
Valve has never SP banned before. This includes Team Fortress, Left 4 Dead, etc... I've been using trainer sense day 1 and almost have SP completed with cheats. Haven't had a single issue.
Valve also gives warnings instead of flat out bans you. You can also contest their bans and they'll listen. At any rate I would still keep caution. There's always some risk given they provide SP achievements.
I however believe it's enabled, but not active because even in SP you're still "online", see Zombie Mode with online play, etc...