Looks like a pretty cool deck overall. I have just a few ideas for cards to include.
Bolas's Citadel: A great way to turn your excess life into tangible card advantage. Citadel can get out of control quickly.
Castle Locthwain: Very little reason to not run it in a mono black deck. Another (albeit worse) way to convert your extra life into cards.
Feed the Swarm: Hey! Enchantment removal in black! Probably worth an inclusion in any mono black deck in my eyes.
Dread Presence: Another card I'm a big fan of in mono black decks. If you need cards, this can do a great job at that, and if you just need to win the game, it can start draining your opponents instead.
Sanguine Bond: Another, harder to remove, Vito seems fantastic to me.
Vampire Nighthawk/
Nighthawk Scavenger: Some more vampires that are evasive and have
lifelink.
Blood Tribute: With just Vito out, this basically says "Kill target opponent," which seems...fun?
Marauding Blight-Priest: Mini-Vito.
K'rrik, Son of Yawgmoth: Similar reasoning to
Bolas's Citadel, just instead of converting life into cards, you are converting it into mana. Also, K'rrik can easily become a big beater that gains you big chunks of life here and there.
There's nothing wrong with your deck, per se, you are just missing some cards I think warrant inclusion.
Cheers!