I don't fully understand how that Cap works from a Vegas perspective. That makes for an awfully top-heavy lineup, given what Marner makes and what he's likely to command on an extension. But it does feel like an entire Vegas move. They're plenty keen to chase the "big names" and have shown as much on multiple occasions.
I'd guess this would effectively be at the expense of letting Marchessault and Stephenson walk. Kind of weird to me, in that their superlative depth was an enormous factor in winning their Cup...and it feels like they're running in the opposite direction. But since the salary doesn't seem to exist for them, maybe they know what they're doing.
From the Toronto end...i think that getting Theodore could be big for them. They could really use his puck moving and transition ability, as well as some PP acumen. Getting him re-signed might be tough and/or expensive, but he's definitely the sort of big minutes defenceman they need, to help keep the ice tilted in their favour.
I don't really think Thompson is the answer in net though. We've seen how interchangeable goaltenders are with Vegas. I don't think just plugging him in with the Leafs is going to completely "fix" things. Though i guess he's probably good enough to platoon with Woll whenever he's not injured.