It already has. Signing Stone put them over next years cap. Plus they still need to resign William Karlsson in addition to 2 more players to get them to the minimum roster size. To sign Karlsson and fill a full 23-man roster, they will need to shed close to $10 million in AAV.
I agree that the capsace will be tight for Vegas in the next year or so, but they still have quite a bit of room to maneuver and probably won't lose anyone too important to them down the stretch. Saying they need to shed 10M$ is a bit of an overreaction in that context.
To clarify, as per Capfriendly, the Golden Knights have 47,125M$ tied in 9 forwards (with Clarksson on IR) next year and Karlsson to re-sign. Let's say he re-signs for about 5,5M$, a low and conservative number I'll admit, they then have 52,625M$ tied in 10 forwards. On defense, they have 21,1M$ for 6 guys and Fleury takes up 7M$, for a grand total of 80,725M$ for 17 skaters dressed.
In order to be viable as a team, and to act as a buffer for injury, Vegas would need to dress ideally 23 skaters, as you've said, and keep at least 100-200k of capspace for short-term recalls and replacement, so that's 6 more contracts that eats at their 83M$ cap ceiling, and with a minimum salary of more than 600k basic math tells us that they would be over the cap.
However, and that's where it gets interesting, if they move Ryan Reaves' contract for next year (which shouldn't be too difficult if they add a mid-round pick or a young prospect), totaling 2,75M for the 2019-2020 season only, they'd then stand at 77,95M$ for 16 skaters and would then have 5,05M$ to dress 7 players. Of course, that's not a lot of room, and they still have to re-sign Gusev and accomodate rookies like Glass potentially making 925k on their ELC, with the potential to hit on their bonuses, so the management of the Golden Knights will have to pay close attention to their cap, but I think it's not something that's too worrying for them at the moment.
Tampa Bay is in a much worse situation to be completely honest, one with no easy fix, and will lose at least two good pieces (probably Stralman and Miller) off their roster, if not more.