Vacuum vs Auto Vacuum

Question

I am trying to find answers for my below few queries which will help me in fine tuning my postgres DB. I did some googling but was not able to find answer.

1) Out of vacuum or Auto vacuum which is more resource intensive operation?
2) Is there a way to run Auto vacuum only when there is unusually heavy update activity?
3) How can I find out based on setting when my auto vacuum will start next time, based on threshold.