i will try to simulate this , but i myself merged 2 vamp cards and all was ok . but did not tested this inside the deck.
edit : when having only 20 deck cards and do a fusing inside the deck it will pull 1 random card from the inventory to fill the missing spot
edit 2 : made a new account to simulate what will happen if you had only 20 deck card and no cards in inventory. Result : you can NOT fuse the 2 cards in the deck , until you get 1 card more in inventory or in deck