Even if Vatican II was heretical, we should still follow it.
If anything is heretical, you shouldn't follow it. Why would you think otherwise?
God will hold the people responsible accountable, not the people who faithfully follow the Church.
God will hold all persons accountable for their actions. What do you mean when you say "faithfully follow"? Follow what? It seems to me that most modern Catholics are really following anything other that their own feelings.
IF it is, then why did God not prevent it from happening?
Because God allows all things, good and bad. Can you think of some other examples of what God has allowed that wasn't an immediate good? Why did God allow the Reformation? Why did He allow the Western Schism?
Vatican II is 100% legitimate in my view.
Even if it was heretical, right?