>>1880179You float if you have lower density than the fluid around you, and sink if you have higher density. To float in water you must be lighter than water and to float in air you must be lighter than air. Usually you would achieve this by having a light substance contained in a shell that prevents the heavier fluid from getting in. Ships float on water because they are full of air and airships float in air because they are full of helium. This is the principle of buoyancy.
A vacuum balloon works on the same principle, but instead of filling it with something lighter than air you simply suck all the air out of it until it has a lower density than the air around it. The drawback is that you end up with a pressure difference of almost 1 atmosphere, which will cause the balloon to collapse unless the shell is very strong and rigid. The shell must also be extremely light, however, otherwise buoyancy will not be achieved.
So you basically need a very thin shell of low-density material that is capable of withstanding 1 atmosphere of pressure without breaking or deforming. You might be able to do this with some kind of exotic composite material, but it likely be very expensive and difficult to mass produce at first.
