I didn't get into the various Maskwood Nexus shenanigans one might get up to with a deck like this, but I'm sure there's something degenerate I could do with the right inspiration
The Harbinger should, obviously, only be played in mono-blue Merfolk decks, and preferably in control shells that focus on icing out the opponent from doing much of anything rather than overwhelming them with tons of creatures, just like Richard Garfield intended
You can't do so later in combat, and creatures put onto the battlefield attacking can't be exerted
Vanquisher's Banner, Coat of Arms, and Banner of Kinship are all artifacts that'll boost your Vampires drastically
Every Forest , Island , Swamp , Mountain , and Plains has an intrinsic mana ability