I can't opine too much about JVR because I don't really know what kind of overall player he is, or how his scoring might translate to playoff hockey (that would be the big red flag -- if there are reasons to believe he's a regular season specialist).
Even if he's a one-dimensional scorer, it's hard to imagine not preferring him instead of Mojo (or Laich, of course). A guy who can score consistently is a valuable piece.
A more difficult question is he really good enough to warrant giving/adding one of the Caps' better prospects. I don't know..
Finally, it's hard to say whether JVR would be a guy Caps would consider because we don't know what their vision is for the 3rd line. Does it have to be Trotz-style heavy? Are they considering a more skilled/balanced top-9? Is getting a good 3C their overriding priority? And of course, availability..
If I could I get JVR for Mojo + a reasonable add, thought that his offense would translate in playoffs, and was willing to experiment with more skill on the 3rd line, I'd surely go for it. Can always get some checking center at the deadline..
But yeah, it's hypothetical, because I think everybody at this point expects BMac to sign MJ + Holtby and squeeze in a 3C if there's enough left over cap space..