How Good is VAC

The reason why people complain about VAC not being all that effective is because Valve doesn't want to make an intrusive anti-cheat. For example there was a huge community dissatisfaction when Insurgency added BattleEye Anti-Cheat (even tho it is used by many games), because it was somewhat intrusive. If Valve made VAC more intrusive, yes , it would be more effective but at the cost of privacy which would turn a lot of people off so they probably will not do it