Another tip I've done for pinpointing vac leaks, the bodge job farmer way. If you don't have a smoke pen and don't want to go buy one, use some brake cleaner (non chlorinated) or starter fluid. Give it GENTLE TINY sprays around vacuum connections and components and see if the engine idle changes. An increase in idle means it's drawing the fluid in and burning it, aka a leak. Do it sparingly and wait a minute between sprays so you don't have a buildup of flammable vapors.

Incidentally this is a great way to also find bad spark plug wires...