It would take a lot more than a 3rd round pick to persuade Vegas to select Orpik over Grubauer. You'd not only have to give them more value in trade assets that they'd want for Grubauer, but also more value in trade assets than you'd be willing to pay to simply trade Orpik to them later. Vegas dosn't have tremendous incentive to eat your high cap hit contracts in the expansion draft, because they have the option to take the player they want in expansion AND then get you to sweeten a deal for them to bail you out of cap hell.
Think of it this way, if you're McPhee which would you rather have? Orpik + 3rd round pick or Grubauer (expansion draft) + Orpik + 3rd round pick (later trade)? The sweetener on top of Orpik would have to be pretty darn hefty for them to pass up taking someone like Grubauer, Schmidt, Wilson, or Connolly.
And Washington already traded away their 1st, 2nd, and 3rd round picks in the 2017 NHL draft. Also the 2nd round pick in 2018, and conditionally the 2nd round pick in 2019.