Just a few quick thoughts:
1) You currently have 5x Vampire Hexmage - it's likely just a typo, but regardless. Further, what're you needing to remove counters from that you would want the Hexmages?
2) I'd adjust the mana mixture as it looks like you have a lot more need for black than red, so a 50/50 mix probably isn't the best ratio. If you enter the deck into deckbox you can click the "tools" tab in the upper right when you're looking at the deck and it will give you a breakdown of the mana curve and a suggestion for how many lands of each color you would want (you can adjust the total number of lands too).
3) There are a wealth of vampire cards out there that would/could be great in here - depending on what format you're aiming for of course. If you're going for standard, I'm not positive exactly which of these are legal off the top of my head, but I know several aren't.
- Blade of the Bloodchief - not standard legal, but MAN is that a sick card for a vampire deck
- Feast of Blood - also not standard legal, but a GREAT removal option. It can help in a pinch with life gain too.
- Urge to Feed - again, not standard legal, but another great option for those pesky indestructible creatures (and a good way to keep pumping those vampires).
- Blood Artist - standard legal, and it will accomplish the same goal as Kalastria Highborn without costing you any mana.
- Gatekeeper of Malakir - not standard legal, but it gives you a 2/2 vampire AND removal in the form of its kicker.
- Sangromancer - standard legal, a great creature as it's a four drop 3/3 flyer that also gives you life gain
- Anowon, the Ruin Sage - not standard legal, but man is he mean... you can essentially just sit back and watch them sacrifice their creatures while you don't sack anything
- Heirs of Stromkirk - standard legal, a good option at a four drop as it pumps like just about everything else you want in the deck, but the beauty comes from having intimidate
- Havengul Vampire - standard legal, and another good option at the four drop spot
I realize I've just listed a LOT of cards, but those are just some of the easier options that come to mind. My suggestion would be to drop some (or all) of the Guul Draz Assassins to replace with something I've listed above. Definitely swap the Kalastria Highborns for Blood Artists.
My only other thought, and this is one where I differ from most people, I suggest dropping to three of a lot of the cards rather than four (with exceptions for ones you REALLY want) as that creates space for you to plug in a lot of other options. I know consistency is normally everyone's top concern, but I don't think moving more toward 3x of most cards reduces the deck's consistency as much as it provides increased flexibility (you create room to have more combos rather than just relying on the same one or two that you would have if you have 4x of everything).
As always, playtesting and your personal preferences/playstyle will dictate how to best proceed, but I hope these suggestions help.
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