Umm... They're both bad ideas?
Vanek the first time around was worth a short term gamble, but didn't net the return most of us expected. Now? After he's in even less demand, is a year older, and Detroit has more promising forwards in the pipeline than before? I'll pass.
Sign the kids you know you want to keep (Larkin, Mantha, etc.). Try to trade for a player young enough to still be useful in 5 years. If you do anything at all in free agency, look for the NEXT Vanek via a slightly younger guy who didn't live up to expectations and might be worth a flier.
Vanek, part deux, feels kinda like Samuelsson. The first signing was a decent stretch. The sequel was a bad idea that became a worse reality. Vanek will either help them win a few meaningless games to drop a couple draft positions, without returning anything at the deadline, or he'll finally hit the wall, and be a dead cap money signing for no good reason.
In theory, it's still better than overextending yourself on a lousy deal long term, but I'd rather skip both options than take the lesser of two evils.