Instead of Doom Blade try Victim of Night or Dismember both are more effective kill spells. Victim can be almost always an unconditional kill spell because hardly anyone runs any of those 3 creatures in modern and dismember can be cast for 1 if in a pinch. Really good

I would remove Clutch of the Undercity for Ensnaring Bridge or maybe Behind the Scenes just to make all your small vamps unblockable. This seems like a fun deck that goes off without much of a warning, great deck

Have you tried going Mardu, Rakdos, or Orzhov instead of Dimir? You can get more removal and lords that way. Some cards I suggest if you want to switch up the colors or just in general:

• Vampire Nighthawk

• Stromkirk Captain

• Mavren Fein, Dusk Apostle

• Legion Lieutenant

Hexaflexagon

If I was going just for vampire tribal I would. However, this is an infinite combo deck of Bloodline Keeper + Intruder Alarm . I should probably update the description to make that clearer.