Is the cap supposed to be vented?

When I remove the cap while the engine is running, the idle speed drops a bit and there is an obvious tight vacuum seal to break to remove the cap.

 

No, lots of engines idle will change when you remove the oil cap. You are basically causing a pressure difference in the crankcase pressure when you remove the cap, which screws with the PCV system, so the air volume going into the intake changes so the idle changes.

I don't think I have ever heard of a vented oil cap personally. Maybe long before they had PCV valves, lol.