There are two types of successful possession WR's.
The first type is a taller, big WR with excellent hands and a large catch radius. A guy that is a big target for a QB to see and dump it off to. Typically this type of possession WR is willing to line up in the slot and move around for matchup purposes and he is willing to go across the middle on crossing routes and drags and absorb the big hit while still hanging on to the ball.
The second type of a successful possession WR is a really quick player. A player that can not be pressed at the line and can get off the LOS quickly and get open. A shifty player with great quickness and acceleration...can get up to full speed quickly.
The one thing that both of these types of WR's have in common is both players tend to lack good deep speed, which is why they aren't playing the X or the Y position. If you have great speed along with the skill-sets i've described, then you are definitely balling out in the NFL or college. Guys that become good possession WR's have important skill-sets, but unfortunately they lack an important one which is speed.
Possession WR's typically have great hands, are unafraid of contact and going over the middle often, and they are reliable route runners that are always in the spots they are supposed to be in.
Just because Van runs good routes and he has good hands does not mean he has possession WR skills. All the good and greatt WR's have those skill-sets, they just also have freakish speed and jumping abilities as well. Van has very good speed and ball skills. He is not going to be wasted in a possession WR role. He will play the X, Y, and Z positions because he has the skill-set to play anything he wants.