"H2O is lighter than air"... "Cold Steam"
Cold steam is water, which is most definately not lighter than air as the weather is proving quite abley at the moment up here.
I did some calculations on this- and to lift a 100kg man (of which I am a frankly awe-inspiring example) you would need approximately 120m^3. Yes, this is probably less than a standard airship- but you've then got to factor in the mass of the envelope itself. This envelope has to be strong enough to prevent itself from collapsing in on itself- and therein lies the problem. It's a pain in the ass to build a structure that won't collapse in on itself on such huge scales with tiny thicknesses of envelope wall.
Vacuum ships, therefore, only work at extremely large scales (according to my kickass spreadsheet on the subject- yes, I have looked into this before and yes, I clearly have too much time on my hands...) and are still pretty useless even then.
A helium ship carrying (or surrounded by) a cardboard (or nice bulletproof Kevlar) cutout of the Battlecruiser from StarCraft would be awesome- even if it was of limited strategic value, just seeing one of those leviathans hovering over you would be enough to strike the fear of God/Allah/other into you.
Turn it into the worlds biggest and slowest UAV so you're not risking personnel and you could even go for a cheap Hydrogen based version!
Strap a few Hellfire missiles to it, a few solar panels on the whole top of the vehicle to keep it energy-self-sufficient for ridiculous lengths of time and build in some sort of UAV-based rearming system for the Hellfire launchers and to re-stock any escaping lift gas et voila! A perfect low-maintainance Shock and Awe weapon capable of wiping out anyone that attacks it.
That's about when DARPA starts looking at building a Yamato Cannon...