Together with my coauthor, I've spent quite some time "engineering a vacuum balloon":-). Our conclusion: difficult, but possible with current materials. Although an evacuated homogeneous spherical shell cannot be both light enough to float and strong enough to withstand atmospheric pressure, no matter what existing material you use (e.g., diamond's elasticity modulus is at least three times too small), a sandwich shell can meet these conditions with a reasonable safety factor. See details in our US patent application 20070001053 (11/517915) (you may google "layered shell vacuum balloons"). I agree that a vacuum balloon will not have greater lift than a helium balloon, but it can have some other strong points, such as easier altitude control.
