Edited Aug. 30, 2018 at 10:21 a.m.
If the Caps keep developing well, and Vrana sticks in the top six role he got into in the playoffs, and won the Cup in, then Vrana scores 20 goals this year. If Copley starts 30 games behind the Capitals' defense this year, then Copley wins 15 games, and the Caps re-sign Holtby long term and can trade Copley for a mid-round pick.
Huberdeau has scored 20 goals twice, but not in consecutive years. Maybe he does it again this year. Maybe he even breaks 30. Maybe he only gets ten. He's decent at puck possession, but Vrana is too, and Vrana is younger.
The value is there today, with Vrana unproven and Huberdeau coming off a career year, but next summer I don't think it will be there, and you're right that the Caps don't consider trading Holtby before next summer, because next summer they get to see where the salary cap is headed and they get to try to negotiate an extension with Holtby, who seems pretty happy in Washington, where he just won the Cup.
I think next summer, with the salary cap being what it is, and the game getting younger, the Caps keep Vrana, but offer either one of Niskanen/Oshie in that trade to make it fair.