A couple months ago I would've said Vanek, but I'm not so sure right now. Vanek's doing well in his first year, but Lupul is looking scary in his second. He has been horribly inconsistant, but when he's been hot, he's been dominant. If he finds consistancy, or even just becomes less inconsistant, he's going to be amazing.
I'd still say Vanek will be better at this point, but, it most certainly will not be by a longshot.