Does your apartment have a 240v stove/range outlet? Or an electric dryer outlet? Because those would be my go-to. Failing that, one of those transformers is an option.
However, as already said, you're only gonna get 1800W out of a typical 15A circuit, and that's assuming NOTHING else on that circuit is drawing juice. If you overload it and trip the breaker, you trip the breaker. That's what they're there for. There's no more fire hazard than there is normally (ie, old building with old wiring), so don't worry about that.
Possible problem #2, you're looking at American to European step-up transformers. The rest of the world (foolishly) chose to use 50Hz instead of 60. I can't find any info on those converters as to frequency. I'm *assuming* it's just a giant transformer inside, which would output the same Hz as is input. Which would be good for your purposes of using 240v American appliances. But if by some chance it was a digital converter, it might output 50Hz, which would not work with most central vacs, as they usually have induction motors (which depend upon the correct frequency).
Now the real question is what is the amp or watt rating of the 240v appliance you're thinking of using? Remember that Watts = Volts * Amps, so 7.5A * 240v = 1800w = 15A * 120v. Basically watt rating is independent of voltage.
Oh also. If you're clever, and I feel like I saw this mentioned in another thread not long ago, you can find two outlets on separate phases in your apartment, and with a little ingenuity (and a trip to the hardware store) you can wire up a 240v outlet. Basically, half the circuits in your place are on one phase, and the other half are on the other. Two phases together make a whole 240v circuit. However, I wouldn't hardwire anything like that, but as a temporary thing, like an 'extension cord' that you unplugged when you finished using it, would be fine. Then, in theory, using two 15A circuits, you could create a 15A @ 240v circuit. Again, with no other loads. Actually, this would probably be easier, cheaper, and even safer than using a converter. Because, again, a converter puts the whole load on one breaker, and would then be limited to half the current draw as the other way.
Speaking of wiring, at least the first converter uses a universal outlet for the output, which would support using a normal US 120v plug. I'm fairly certain most central vac units would not even have a plug, being hardwired. But that's easily wired to a cord and plug. When you start getting into stuff like using a 120v plug for a 240v appliance, I would really make sure you label everything, so nobody does anything stupid. Not that plugging a 240v motor into 120v would do much.
You know what you need to do? You need to find the breaker box for your apartment and look inside it. Maybe snap a pic for us. Sometimes the handwritten labels would tell you enough. ie, one of them says 'dryer' or 'stove.' But yeah, definitely also let us know the power ratings of the stuff you wanna use too.
