Victor, I'm no scientist, but it's even blindingly obvious to me. Origins clip - the air in the flask is sucked out - it becomes thinner. The air in the balloon, now having more pressure, thins to be in equlilibrium with the air in the flask.
Think of it this way. Imagine there was no balloon in the flask - just an imaginary one. The air in that volume bounded by the imaginary balloon thins out .. behaves the same way as if the ballon was there. Because the ballloon is soft and maleable. If it was a steel hard sphere it wouldn't.
Your so called Veegrans .. or whatever, behave the same way. Think about it for a minute before you decide to dig your heals in.