VAC Detection

I have multiple CSGO accounts that I use, and I have used external hacks before that I have used. After one account got VAC Banned I deleted the hack and then stopped using the clients for a long time. Now I am going to try it again.

I am just confused a bit on how VAC Detects these hacks on your computer, if the hack does get detected, does VAC base if off of if it is on your computer at all, or if it is just running?