the answer is a subjective one. I know some teams probably will scoff at Vanek just because of his name. Whereas some owners will see the current value in him for a playoff run. The bonus with him is he hits both wing positions and PPP's. Two keys in moving a player. I would think if you can get a Meier/Eriksson Ek type player, maybe even an A. Nylander/Jost from the right motivated owner. I would rule out the Puljujarvi/Dubois prospects.
I would think Meier-esque would be a decent prospect return for him. Otherwise, you could also look at underperforming players that are a year or two removed from their draft year and not hitting expectations - Bennett, Duclair, Larkin.
There's also the outside chance you can get two owners bidding on him and get a higher return. I just wouldn't expect it.