OK, I'm not sure if this is what you want at all, because you say 'not what makes it VSF as opposed to other genres' and I'm using other genres as a point of comparison, but I think what makes VSF VSF and not Pulp, Colonial Gaming or 'hard Sci-Fi' is important (that's not to say that I think VSF games can't use Pulp, Colonial or Sci-Fi rulesets with a bit of creative jiggery pokery).
I may be being a bit heretical here, but to me what makes things VSF (rather than say Pulp) is that the heroes are called Captain Cholmondely-Carruthers, not Zac Smash; and dependable old Blenkinsop is as likely to be a Martian/Clockwork rather than some chap from the lower orders. So the ability to have characters from any of the innner planets of the Solar System (I don't think we really need jellyfish-gasbag Jovians and whatnot, but Selenites, Venusians and Martians certainly, both as heroes and as troops), constructed entities, or certain kinds of alternative-earth people (eg Atlanteans, Insect-people from Inside the Earth or whatever) would be good. Oh, and I would certainly regard 'characters' as vital, if by characters you mean the ability to put certain models on the table with a changed or abnormal statline or abilities to represent their different capacities.
Obviously what seperates VSF from Colonial Gaming is the 'weird science' element, which you've already covered (but see below). But VSF should be able to do asymetric warfare, whether that's 100 Zulus against a small detatchment of Armoured Flux-Bombardiers, or a horde of Transylvanian peasants against Dr Grimlichfienden and his Glavanic Titan, or even a small party of intrepid explorers going through the caves beneath Iceland and encountering weird beasts and troll-men on their Travels to the Heart of the World. I know that is pretty Pulpy too, but to me Pulp is a bit like VSF 30 years on.
I'd say what seperates VSF from more 'hard' sci-fi is the experimental nature of the creations. I know you've said that it's covered, but I think there's a really important point here, so I'm going to persue my angle on it. VSF is to me a cusp of history, when technology is moving from the time of inventors in their sheds to large companies with big factories. The idea of breakdowns occurring should be built in. I'd propose that it's not individual objects that should break, it maybe could be types of object - if you have a Colossus Steam Walker, and a patrol of 10 cavalrymen with Galvanic Lances, the Colossus, being a jerry-built one off that Professor Parpington built in his shed, should be inherently more unreliable than the Galvanic Lances of Imperial Galvanic Engineering Ltd of Hull. Perhaps some sort of 'tech level' might be necessary (the further from the norm the more chance of breakage) or some relationship between how many of something there are, and how reliable they are, to reflect the difference between craft and industrial methods of production.
Beyond that, dinosaurs and airships; if it has those, I'll be happy.