PC users with the follower mod such as myself can take Serana and other followers. Been doing that myself.
However for console players, I believe if you have a follower with you when you find Serana, you get two followers until you return her to her father and she leaves your company for a time.
This makes sense. I guess the three day separation when you cure her does it. I guess if I want the benefit, she stays vamp. This particular trio works well, as they prefer ranged attacks and stay out of each other's way and avoid friendly fire. In character, I would rather see her cured, she seems to welcome the change, but I will be selfish and take the two followers. (it would be nice to have Serana oddly classified, and allow you to have a second follower with her. Console users can't get the, well -you know....)
Off topic, it's about time that I give you kudos.... Your screenshots are, quite simply - amazing - as is your 'wrecking crew'! Though I'm stuck with an Xbox, I applaud the pc community on what they have accomplished. If I ever det the disposable coin, I may build a rig that I can hook up to the big flat screen with all the benefits of hi-rez!