Yeah, having to use a vacuum toilet would totally suck. Ba dum tsh.
But as for the free-floating pee problem, that should be easy to solve, I think. Pee is forcefully expelled from the bladder, so you just need to keep it in one place after it’s out. I’d think of a disposable potty with some sort of absorbent cloth inside that soaks up all the urine. In other words, a sponge in a bottle.
By the way, this question made me wonder whether peeing while upside down would be very different from peeing downside down. But I don’t feel particularly enthusiastic to give it a try.