I'll just add my two pence worth. I do not have the experience with using but making valves. If you are looking at 1 & 2" valves, I would suggest that you are looking at 3 piece, bolted construction? These are generally rated anywhere from 60 to 138 bar, so the pressures in a vacuum system are not going to get the floating bit happening to any great degree, you will really be relying on the crush of the seats. Therefore I would be more concerned about the environmental seals, these are generally designed to stop the pressure getting out rather than in. The same goes for the stem seals, generally the pressure is pushing up on the skirt of the stem, often assisting with the sealing. Not in this instance, it isn't! Therefore I would suggest that you look for pure PTFE seats and seals - I assume that fire safe is not a requirement? Ideally the body seals should be a 'tongue and groove' type to ensure adequate compression on the seal and to stop it moving (in!). I would also suggest that a stem with a stem nut and spring seal energizer would assist, this means that you can ensure the the lower stem seals remain in contact with body and stem, no pressure related forces required. Lastly the surface finish - and fit - on the stem is quite critical, the more highly polished the better, but not at the risk of taking it out of round.
I take the experience that ZDAS04 gave, viz once you loose surface finish on the ball, game over. So this gets back to how often you operate the valve. In a high cyclic operation I think that you will be changing out valves on a regular basis. The crush on the seats will not last for ever. You could look at harder seat materials but that will also make getting the seal more difficult.
Good luck, be interested to hear how you go!
