This sounds way too powerful to me. Too good in engines. Also, neato thing:
I play fortress.
I play vassal, trashing fortress into hand, gaining BoM on top of my deck.
I play fortress.
I play BoM as vassal, trashing fortress into hand, gaining BoM on top of my deck.
I play fortress.
.
.
.
Now I have 22 coins, 1 action, a fortress in hand, a 5-cost on the deck of my choice, and all 10 BoM. Okay, it's a 3-card combo. Whatever.
In general taking cards that you put in play out of play is too good. The only published cards that do this take themselves out of play or are procession, which itself is really strong.
Also, I could empty the supply in 3 turns with this card.