2011-01-22 : You can tap any untapped Vampire you control, including one you haven’t controlled continuously since the beginning of your most recent turn, to pay the cost of Chosen of Markov’s activated ability. You must have controlled Chosen of Markov continuously since the beginning of your most recent turn, however.

2016-07-13 : For more information on double-faced cards, see the Shadows over Innistrad mechanics article (http://magic.wizards.com/en/articles/archive/feature/shadows-over-innistrad-mechanics).