The tradeoff is being able to burn a blocker and play another vampire on turn 4, vs. being able to finish off an opponent who stabilizes at exactly 3 life and kill 3-tougness creatures. Since you have a lot of 2-power attackers, killing 3-toughness creatures is more important than usual.
Oh, and while I'm at it, you should split Doom Blade and Go for the Throat to reduce the risk of drawing multiple dead removal spells in the same game, and to give yourself more post-board options.
You are planning to win on tempo a lot of the time. If you waste your first turn entirely, you are already behind. Fume Spitter is special because it's something you can do on turn 1 that helps your vampire connect with their face on turn 3.What I don't see is what Fume Spitter will do for me, and Infiltration Lens is nice, but is it really worth including?
Infiltration Lens may be better in the sideboard, but it does tear up opponents whose gameplan is 'block the vampires so they don't get counters' rather than 'kill all their creatures with spells' or 'win faster'.
If all your vampires have +1/+1 counters on them, you are probably winning (or have no vampires left). Undying Evil is for when you are not winning (including saving your first vampire), or for protecting a turn-5 Bloodline Keeper.And Undying Evil is useless, most of the Vampires will have +1/+1 counters.
At the very least, it's something you should try. A dozen matches against a varied field, with 2-3 copies maindeck should give you some idea of how well it fits in the deck.
In that case, you need more things to do on turn 1 (see above).And Duress will have to be cut, because I do want to have at least one standard deck.
The problem with extra Blademasters is that they strain your manabase. Extra Keepers are justified as a solid plan B; you can use them to come back from behind even if you don't get two Swamps until turn 8.How many of my other vampires should I be going for? Should I go for four Keepers and Blademasters, or leave them at two and three?