Not possible. Once she's cured, that's it. Only other option is to start a new game, if it's that important to you that she stay a vampire.
click serana in console commands, then type:
Setrace nordracevampire
Evil Cicero says: "Go on listener, use the cheat. Uuuuuse it! It's right there. So easyyyy. So what if it's cheating, USSSE IT!"
Nice Cicero says (into your other ear): "Start a new game, Listener. It's more fun. Less regrets! Think of all the mayhem you can get up to AGAIN, and get your follower/minion the way you want herrrrr. New GAME-M-mm-M-mmmmm! Do IT!"
Now which Cicero are you going to kill? I mean listen to. Which?