Lifting gasses provide two things: lift (of course), but also structural integrity: their desire to expand reinforces the structure containing them. Vacuum prov

Lifting gasses provide two things: lift (of course), but also structural integrity: their desire to expand reinforces the structure containing them. Vacuum provides slightly more lift than helium but also provides negative structural integrity, since the atmosphere is actively trying to crush the vacuum chamber. From all accounts it would be a seriously challenging materials science endeavor to build something that could withstand containing a vacuum without adding more weight than the gain in lift, so in practice it would be very difficult to realise the 16% lift improvement. One method is to go bigger: because of things like the square-cube law, the lift provided by a vacuum container increases faster than the demand on structural integrity, so a sufficiently large container lets you use more conventional materials. It’s been a long time since I did the math and I wasn’t very confident of my calculations even then, but I vaguely recall if you used steel you’d be looking at containers with multiple cubic kilometers of volume or something - highly impractical. Another approach is only using vacuum in the upper atmosphere. The pressure on the vacuum container is much lower up there so you would have less of a structural integrity challenge. You don’t gain as much as you’d think from this approach, however, because the positive structural integrity provided by the expansion of lifting gasses allows for containers that expand, increasing their volume in response to lower air pressure (we call these containers “balloons”). In fact, because of this property, to a first approximation altitude has NO effect on the lift provided by a set amount of lifting gas (counter-intuitive, I know), and getting a helium balloon to STOP rising is a real challenge. Getting the vacuum chamber up high enough to make it safe to pull a vacuum is obviously a bit of a chicken-and-egg problem too, not to mention you’d have to bring the extra weight of a vacuum pump with you. Where this idea might work is if you’re coming in from space in the first place. Assembling the container in space means you don’t need a vacuum pump, and since you’re descending from above you don’t need a container strong enough to withstand containing a vacuum at sea level pressure. So this might have some use for putting floating structures into Venus’s upper atmosphere.