What would happen if you could fill a balloon with a vacuum?

It would suck in on itself. It would not maintain its shape.

Yes. The reason that a balloon floats is because the weight of the balloon is less than the weight of the corresponding volume of air. Since a vacuum weights nothing there would be even less weight to the balloon than when it is filled with helium. This is of course assuming that the balloon would hold its shape, which a normal balloon wouldn’t.

Air pressure at sea level is around 13.6 PSI. That doesn’t sound feasible.

Yes, as part of the premise of the question, you are able to get the balloon to hold it’s shape.
Since it would seem to float faster and higher, how hard would it be to do that?

Ok. If you could find a way to keep the ballon shaped it would shoot up rapidly. Shellac it when it’s blown up?

@Adirondackwannabe Wrong number; 14.7 is what you want ;)

A perfectly spherical shell of thin metal might be strong enough to withstand crushing by atmospheric pressure yet light enough to float. I’m not sure you could make it as small as a toy balloon, but it scales up nicely. When you double the diameter the buoyancy increases 8-fold while the weight increases only 4-fold (a little more, because bigger means stronger means thicker material).

It wouldn’t take much of an air leak to bring it crashing down, however. You go first!

@gasman I can imagine. One small structural failure and the balloon instantly crushes itself. Wouldn’t want to be there when that happens.

I wouldn’t use a latex balloon, since they would try to contract just from the stretched latex.
Perhaps a mylar.

..It would look like a vacuum in a balloon and would be worthless for sweeping the carpet.

Higher air pressure always tries to transfer to a lower pressure so the balloon would implode. A vacuum ‘leak’ is really higher air pressure trying to take up the space of lower air pressure (I know this from working on industrial vacuum systems). Hypothetically the balloon would float.