<<Perhaps you want to re-evaluate your terms? "Atmospheric pressure" is pressure related to the atmosphere. "Partial pressure" or "vapor pressure" is the term you're looking for.>>
I don't see it making much difference. The atmospheric pressure varies with location, it's one thing in the Mojave Desert and something else on top of Mount St. Helen's. I was talking about the atmospheric pressure inside Stella's cup, which references a relatively small atmosphere but IMHO an atmosphere none the less. But by all means we can call it vapor pressure inside the cup.
<<And when you release the lid holding in the vapor pressure, the pressure will only lower to atmospheric pressure - which is unchanged.>>
Well, in a previous post in this thread, I said that the release of the lid would bring the vapor pressure at least down to atmospheric pressure, but possibly lower if vapor and gases inside the cup are sucked out in the wake of the exiting mixed gases, kind of like the Bernouilli effect in a coffee cup. Admittedly speculative but not IMHO impossible.
