Correct. If it's questionable, the league will probably not do much if anything look at it just to make sure it passes the sniff test.
Team A can flip a player to Team B. Team A just can't get the guy back until after 1/31/22. Team B could even be Vegas and the flip could be before the Expansion Draft, but Vegas will (A) have to have the cap space for the guy now, and (B) have to have the cap space for the guy during most of '21-22, which might (will likely) be an issue.
Team A can flip a player to Team B (not Vegas) before the expansion draft for Team B to protect. Again, Team A still can't get them back before 1/31/22 and it means Team B has to leave unprotected someone it might really want to protect. Doubtful any Team B has a roster of guys and thinks nah, we only want 3 or 4 of these guys. Including Buffalo.
Team A could send a player to Vegas, who holds the guy through the Expansion draft and then sends the guy to Team B. Again, Vegas has to have the cap space for that guy. Given the offseason cap rules won't go into effect until the date that UFA opens, Vegas is going to be cap-strapped as it is so it won't have space to take on guys - but pretending it did, Team A still can't get the guy back from Team B until 1/31/22 so Team B has to have the cap space to sit on that guy. Some Team Bs might have the cap space for it, but even if they do if a guy goes back to Team A between 1/31/22 and the '22 trade deadline, you can bet the league is going to be investigating those pretty closely.
It's a lot of noise over nothing, IMO. The opportunities to take advantage of Vegas are going to be pretty much nil because of how the cap will be applying at the time.