I generally don't use Serana, for a number of reasons. But for the times in the Dawnguard quest where she insists on coming and won't take no for an answer (Gee, that's fun) I put her in Roadstrokers's Rogue-Sorceress armor. Perfect for a light-armor battle-mage.
She is pretty, though. I figure she doesn't really look (or act) like a vampire, so she might as well stop dressing like one.