The general idea for this deck looks very good, but I would cut the Vampire Outcasts. They're very expensive for being 2/2s, so do you really think you can guarantee that the bloodthirst will work? Sengir Vampire might be better, or even Bloodrage Vampire to fill in your 3 mana slot more.
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I'd put the Spirit Mantle's in the sideboard and add Slaughter Cry and/or Flying Leap instead. Timely Reinforcements is a great card for a weenie deck too.
I agree about the Knight Exemplars, but keep in mind that they'll get killed when you do Day of Judgment, unless you have two of them out at once... Even so, having several knights besides an Exemplars out, then killing it and all opponent's creatures, then being able to swing with all your other knights on the next turn isn't that bad at all... Adaptive Automaton might be a good addition too, especially after Knight Exemplar is out of standard at the end of September.