Vegas is up against the cap and they have several big contracts already locked into the salary cap. This is making it ever more unlikely that they will be able to sign Theodore. Due to this, it's likely that this will be Theodore's final year with the Golden Knights, and he will almost certainly test the free agency market next July.
Theodore is about to turn 29 soon, which places him still well within his prime. Due to this, and the tremendous experience he has, you can be sure that he will receive a ton of interest from teams in free agency. And who knows, if the Golden Knights are not in a playoff spot come trade deadline, perhaps they will look to trade him in a blockbuster deal to avoid losing him for nothing next summer.