If you don't know about the glitch with Serana it just is when you use drain life ( I think that's what it's called ) In vampire lord form it decreases its damage with her as your follower.

My question is, is there anyway to stop that happening because i really like Serana but I also really like the vampire form

Also you can have Serena following you with someone else when you first get her to bring her back to the castle

I don't think there's a way to stop it yet. There are other ways to do damage in Vampire Lord form though. If you do side missions for the Volkihar vamps you will eventually get some rings and necklaces that'll increase the effectiveness of some of your skills.

For example, I'm now wearing a ring that lets me summon 2 gargoyles instead of one. It's mega useful.

You could always stick to fighting in melee form though, if the damage reduction is huge.

Off topic, but are vampire lord spells affected by any ot the schools of magic? An example would be is summoning a gargoyle affected by having any perks in conjuration?