As the title says. Is there a US-version of a Vintorez (subsonic, accurate to several hundred meters, semi, breaks down into a briefcase)?

There are a number of .300 blackout rifles but all are pretty big/not sniper-rifle accurate.

.300 blackout rifles are typically designed to be small PDW type weapons so extreme accuracy isn't really the design goal or needed at all for it's intended usage. With that said I'm sure you could build an accurate .300 blackout rifle if you wanted to. Even with supers it's not a flat shooting round at all so beyond about 200 yards the holdovers start becoming extreme, and of course the shorter you go barrel wise the more you exacerbate it. It's just a poor cartridge all around for long range. I don't think I've ever personally seen a .300 blackout rifle shoot 1 MOA.

The Vintorez shooting 9x39s 250-275gr subsonics would have the same drop issues as the 300 blk shooting subsonics. So if your trying to duplicate that then the blk would probably be a close match albeit with a lower bullet weight. If you stepped up to 338 spectre you could run 250-300 subs and probably have better long range ballistics and still be in the ar15 platform. 8.6 blk is something else to consider but then you're into the ar10 sized gun unless you had a barrel made for a pof/sfar.

The only VAL/ VSS rifles I've seen were on Youtube and they were built from kits.  I'd imagine a SIG MCX would fill the role with a takedown stock and easily removable barrel.