Have you seen this PW before, or used it yourself? At 3 mana, she is fairly cheap. There are two options:

• +1: target creature gains vigilance and reach until your next turn
• -2: Exile one of the top 3 cards of your library face down. If it's a creature, you can cast it from exile even if Vivien is gone.

Her passive: You may cast creatures as though they had flash.

Of course, she doesn't make much sense if you have lots of noncreature cards. You cannot choose to not exile a card for -2.

But since many decks are heavy on creatures, why isn't she used more?

Stashing creatures into exile is nice. AFAIK they are safe from whatever your opponent could do, until you choose to bring them out.

Casting creatures with flash though, is literally a game changer. It essentially gives your creatures haste. Makes countering them much harder. And dodges sorcery-speed removal, if only for one turn. And of course, you can destroy combos by having a rule-changing creature jump in at instant speed (like Elesh Norn, Mother of Machines, which disables entering effects for your opponents)

Other creatures become much more versatile and powerful through V, CotW. Maybe the best example is Yorion, Sky Nomad (on enter, exile any number of other permanents you control and return them at the next end step). If your army would get wiped, or Vivien herself removed, flash Yorion in, pack all your stuff in a neat box, and unpack everything once the threat has gone. If you can return Yori to your hand, that's similar to a reusable Teferi's Protection.

Also worth mentioning, opponent's greed and combat tricks. Most people will attack Vivien with just enough to get her killed, and otherwise attack your life. Between declare attackers and declare blockers, you can throw in more creatures and completely screw their attack.

So ... cheap, versatile, powerful, and kind of unique. Is she underrated or what are your thoughts/reasons for why she shouldn't be used?