Weird, but useful.
TL:DR, if tanking is your thing, this is now in your deck. Period.
Edit: By tanking in this review I mean the classical term used for roleplaying and videogames, a character who interposes himself between friends and danger and does everything he can to take on as much pressure and pain as he can handle, so that everybody else might do their jobs in peace.
Certain characters are really good at just, taking pain. That's what they do. They'll even do it intentionally for one reason or another. Obviously the characters I speak off are the + combinations Tommy Muldoon and William Yorick. Leo Anderson can do it too but he's not quite so good at it, those other guys don't care if their soaks die, Leo Anderson cares a lot!
Via beefy allies like Guard Dog, Madame Labranche and cheap soaks like Leather Coat and Cherished Keepsake these guys can take twice as much pain as anybody else and keep coming back for more. They can even leverage it into game advantages by taking on the pain for others, with cards like "Let me handle this!" or Heroic Rescue.
Solemn Vow is the new capstone card to this strategy. The key issue with the other cards is that they only apply when the cards are drawn and enemies attack, raw damage and horror however, those are easy as pie to find, and Solemn Vow makes it possible to just funnel all of that bad stuff onto the tanks and their soaks. You might even have a deck totally dedicated to the funneling and management of damage via soaks and healing, using First Aid and/or Painkillers to move the hurt around and make it go away on a choice soak or heal-able target.
It's fast, it's free, you can adjust the number of copies in case this strategy is important, it has 2 icons so the unused copies double as a defense mechanism for the tank himself.
So yes, Tanking, now a far more viable strategy than ever. Even if the "selfless tank" events we've got already didn't exist Solemn Vow single handedly grants viability to this whole strategy.