A cap-compliant trade needs to be completed on the spot. It can not include a time-delayed component.
Team A can certainly send a player to Vegas before the expansion draft, and Team B can send another player to Vegas before the expansion draft, in return for some agreed upon compensation going from Vegas to those two teams. Those trades are absolutely fine. Those players are now Vegas Golden Knights.
Following the expansion draft, Vegas could elect to trade those players to any other teams, including to those two teams in question. That would ordinarily be just fine... although in this specific case, Bill Daly has already come out and stated that the league plans to watch Vegas transactions like a hawk because they are exempt from the expansion draft. The league could always decline to approve trades involving recent Vegas acquisitions, and probably would prevent Vegas from any such shenanigans this year.
But in a general sense, the point here is that trades with components moving at different times are not permissible. Those are two separate transactions. They are not linked, not formally and not in the form of a contract/transaction kept in a desk until the appropriate moment. Any contract/transaction in a desk is the very definition of cap circumvention.
When the expansion draft is over, if Teams A and B come calling to complete such unofficial trade terms, Vegas is under zero obligation. There are no incomplete aspects to these two trades that Vegas still has to follow through on. These are not future considerations. Vegas is fully within its right to say "I do not know what you're talking about; such informal trades are illegal and you can hardly expect us to help you circumvent the cap." Vegas can claim ignorance, and Vegas would be absolutely right. Neither Team A nor Team B can complain that Vegas is reneging on a type of trade that is inherently cap circumvention.
That's the deal with this idea. The intermediary is under no obligation because trades can not have this time delay. You are trading a player to Vegas. You are then going to Vegas after the expansion draft and seeking to acquire another player. The key is that Vegas is understood to be in a position to double dip, to treat these moves as the completely separate transactions that they are, in point of fact.
TL;DR: You can't park players. It's cap circumvention.