Without knowing who the Capitals are going to extend to new contracts, it's hard to know who to protect. While it's certainly possible Vegas uses a pick on a UFA-to-be, I find it far less likely than them picking a player they would have team control over. Especially given the requirement Vegas has to pick 20 players currently under contract.
So the 2017 UFAs are Alzner, Oshie, Williams, and Winnik. Probably a no brainer that Winnik is left unprotected. So that leaves Alzner, Oshie, and Williams as players the Capitals could gamble with leaving unprotected (assuming none are signed by that point) in the hopes Vegas passes on them in favor of players with team control remaining.