I cast an instant or sorcery and want to copy it with {Increasing Vengeance}. Do I play {Increasing Vengeance} before I pass priority, putting two spells on the stack, or do I wait until after I have passed priority for the initial instant or sorcery?
You keep priority. Vengance has to resolve first before you get a copy of course, so you'll have to pass priority then.
Confirmed.