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My guess:
The balloon inflates because the pressure inside is greater than outside. Then some air is lost through the balloon material itself or the seal at the neck. Then it cools down and the amount of air inside is reduced compared to when you started so you get the effect you've seen.
Cupcakes-
Then it cools down
By what mechanism? We've generally agreed that dilute ethanoic acid and sodium hydrogen carbonate reacting to generate carbon dioxide gas is a net endothermic process, so the "inflated" condition early in the process is below or at best at ambient temperature. We know that in the inflated condition, there's a pressure and concentration gradient across the balloon membrane driving diffusion, so the pressure in the reaction vessel should slowly equilibrate to ambient.
In the case of a butyl rubber balloon, the rubber should dissolve carbon dioxide, which might generate a pressure slightly below ambient since the reaction vessel gas volume would be highly enriched in CO2 before being sealed. this would rely on solution of CO2 in rubber happening faster than diffusion of atmospheric air back into the reaction vessel to replace it. We need more measurements, at least time plots of temperature and pressure inside and outside the reaction vessel, and maybe the composition of the gas in the reaction vessel.
gbj_tester
Why has the balloon gone inside the bottle?
I did an experiment with my kid. We put vinegar and baking powder in a bottle and the balloon inflated. Which I expected. The balloon went down. Which I expected.
But now a vacuum has been created.
How come?