I suspect that if Vanek was getting offers from elite cup contenders with a salary remotely akin to what he has got in the last 3 years, he would jump a the chance, especially with a 2 year contract. But his best playoff year was 12 years ago, and he's (with some justification) got a rep as someone who doesn't bring his A game in the playoffs. He's not a particularly demonstrative personality, isn't the best skater, and has always been a bit one paced in his game, so short of changing his physiology and his game entirely, there isn't a great deal he can do about that.
Basically, he's taking the best offer he can get, based on a range of criteria, sporting/financial/family/comfort. He likes Detroit it seems, and he's been a decent if largely incidental FA for us. There should be no malice on either side - and people telling him to retire (unless they know him personally) are idiots. I'm not going to fulfil my greatest career ambitions, but I make a living doing something I love, giving happiness to others and helping my family. Should I jack it in because my professional ambitions are balanced with personal responsibilities and lack of opportunity?
Lets just agree that re-signing him should be somewhere near the bottom of the Red Wing's list of desirable outcomes. i.e better than nothing, but not by much. Likely, he's done here. He did ok, but on balance, meh - his two times here got us a draft pick that appears to have been not used wisely.