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Here is what I would do:
Focus on aggro.  Keep the lands down to 20, and the majority of spells 3 or less with lots of 1's and 2's.  Drop liliana Vess , too expensive and doesn't do enough the turn she comes out.  Drop Demonic Tutor -- for aggro, every card needs to be a threat immediately.  Therefore, there is no reason to search for a card because all your cards are threats anyway.  Plus, they're ridiculously expensive.  Drop hymn to tourach .  Great card -- even better if you pull off T1 Dark Rit --> hypnotic specter , T2 hymn. But that's a different deck and I'll repeat what Terti said, hymn is best combined with other sources of discard.  Drop Dark Rit.  Again, Terti is correct, no need for it when we're keeping the curve low and fast.

Add vampire lacerator , stromkirk captain , olivia voldaren .  If you need more 1-drops on top of the lacerators, add guul draz vampire .  Also, max out gatekeeper of malakir .  Key cards for this deck should be stromkirk captain , captivating vampire (both boost other vampires and remove opposing creatures).  vampire lacerator and bloodghast are the workhorses.  Olivia Voldaren and gatekeeper of malakir provide additional removal.  Olivia and Vampire Nocturnus can split slots -- they're both awsome.

If you really want to keep dark depths + vampire hexmage combo, the hexmage is still a pretty good vampire, but I'd stick to a lone copy of dark depths.  You can't really go much below 20 lands, and if you don't have a hexmage, dark depths is dead.

Blade of the Bloodchief with some sort of sacrifice engine like Viscera Seer, Bloodthrone Vampire, or Bloodflow Connosier(spelling?!). If you want more aggro go with the connosier.

OK, with the exception of Olivia Voldaren , there is not a single card there that will benefit from more than one Jet Medallion . I've played both extensively in this type of deck. I think (from LOTS of experience, KNOW) in this type of deck, with the possibility of first turn Vampire Nighthawk s and kicked Gatekeeper of Malakir s, and also looking for mana later to level up the Guul Draz Assassin , Dark Ritual is the better bet. It is the risk of 2-for-1, but it will not matter in this deck. Just simply do this: Put the deck together, play it with 4 Medallions, then swap them out for 4 Dark Rits. You'll see a marked increase in speed and success with the Rituals, even if you do lose a vampire here and there.

As far as more instant/sorcery removal, Drop the Guul Draz Vampire for simple Terminate s. Don't let this deck get watered down with funky combo tech. Keep it simple. Malakir Bloodwitch is better than Olivia here. Olivia too slow. Again, try them both, see which you win more games with.

Another key to this deck is the ability to keep that top card of your deck black for the Vampire Nocturnus . The two best ways I've found are Liliana Vess and Necropotence .

In the land dept., look into cheaper duals if you can't afford the big guys. Dragonskull Summit is pretty cheap these days. Other decent options are Tainted Peak and Sulfurous Springs . The life you'll gain through the Nighthawk will make up for the couple of times you lose a life to the pain land. No black mana on turn one spells certain doom for this deck.

I know I'm the only one supporting Dark Rit here, but please just do yourself the favor of trying both and you'll see why I support it so fiercely for this deck.

2 jet medallion
10 mountain
10 swamp
4 dragonskull summit
4  captivating vampire
2 guul draz assassin
4 vampire nighthawk
4 stromkirk captain
4 bloodghast
4 gatekeeper of malakir
2 bloodline keeper
4 terminate
2 dark ritual
2 liliana vess
2 feast of blood