Plekanec could absolutely help the Caps win this year, but not as much as Beagle and, with luck, Connolly.
Pass on Green. Tried him out for a decade. Some good years. But on a third pair, the Caps need someone for defensive zone starts. Carlson and Niskannen are as good with a puck as Green. Niskannen is better, defensively. Don't have space for him at anything over 1M, and wouldn't give more than a 3rd pick. Plenty of other teams could see a huge increase in their offense, and would give the above and more for him. The Caps wouldn't, just now
Vanek, absolutely.
Also, why is Plekanec, at thirty-five years old, on pace for thirty-two points, slotted above Eller, twenty-eight years old, on pace for 40 points?
If it's because the third line isn't supposed to score a bunch of goals, like Eller's line does, and Plekanec takes only 40% of his faceoffs in the offensive zone and wins 53% of them, then the Caps would probably just keep Beagle, who takes 26% of his draws in the offenzive zone, but still wins 57%.
https://www.hockey-reference.com/players/e/ellerla01.html
https://www.hockey-reference.com/players/p/plekato01.html
https://www.hockey-reference.com/players/b/beaglja01.html
Plekanec could certainly help the Caps at 4L, backing Beagle up on faceoffs, and killing penalties. he might even work his way up to 3L, while Vrana is in the minors for salary cap space, if Burakovsky stays in the doghouse. I think the rental value is probably a 3rd pick.