I'm really interested in B/W Vampires as a general archetype for post-rotation. I don't think it's going to be tier 1 until Rivals of Ixalan, but I think it looks like a very fun deck. Your creature package looks really good here, I hadn't thought of including
Crested Sunmare. It's a nice payoff for all this lifegain, and I like that you have
Vona, Butcher of Magan as a way to make even more use of all this extra life we will have.
Sanctum Seeker doesn't seem to great to me, but at the same time I can see how it might be. I'm definitely going to play with this deck later tonight and I'll let you know how it performs for me.
Personally I think the only area your deck could use some cleaning up is the noncreature spells. There's a lot of natural lifegain already through our aggro plan here, I don't think
Ritual of Rejuvenation is going to be at all relevant. I understand the appeal may be more the fact that we draw a card, but if you're looking to draw card's I think even something like
Arguel's Blood Fast might be better, as it's another way to capitalize on all of our life gain as well as provide a consistent draw engine. I'm a little unsure about
Oketra's monument as well, though having two or more in play at once could yield some very sweet results, so I do understand its place in this deck. I think bringing your
Walk the Plank up to 4-of would really help this deck out, as
Bloodcrazed Paladin and
Yahenni, Undying Partisan both make us want to be killing creatures very much. If only that card was and Instant, it'd be so damn good. I also think
Call to the Feast seems pretty good in this deck, as it looks like we're heavily rewarded for going wide and gaining lots of life.
Overall the deck seems pretty sweet and I think it's much better than my own prototype B/W Vampires build. Definitely gonna check this out later when I have some time to play. Have you been able to test it at all?