There aren't really any dedicated vac guns... Well, that's not entirely true, of course and I know what you are saying - that some guns come equipped with a vac muzzle already. But apart from the muzzle, the internals on all these offerings are no different from a non-vac gun. So yeah, just slap on your fave vac muzzle and make pretty much any (Italian style) airgun into a vac gun. That said, I did buy a much hyped One Air years back (didn't know much, haha) and these days, I still find a Predathor Vuoto quite good value for the money (I know more now). But the truth is, the Predathor wont shoot any better than a 30 year old Sten with pretty much any after market vac muzzle on it.
Sadly, I can't help much with your STC question as I only have one of their first gen muzzles and those were a bit heavy and pricey. They have since lost weight but are still pricey and use proprietary seals.
For a shortish gun, if I didn't just go for easy - which in my book is a Vuoto muzzle - I would personally probably go with a Tomba. It is cheaper and super well made and you get something almost custom - while still using regular o-rings. The one main difference, I think is that the Tomba - in its current form - needs to be loaded out of the water but Tomi explains how it's a simple operation that just takes a few secs and I am sure it's probably easy unless the gun is long.
BTW, we went over the available off-the shelf muzzle options in this thread not so long ago (so, no Tomba there but Tomi/Marko could tell you if Marko has time to make you one these days):
I keep falling behind with this question but I was hoping someone could list any vacuumed or dry barrel guns that can be bought off the shelf....
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