I am playing through dawnguard as a vampire lord but at some point wish to do the companions questline i know I can't be a vampire and finish this series as I need to be a werewolf. My question is.

Can I finish Dawnguard quests and then do the companions do that series of quests and then revert back to being a vampire lord?

I prefer the VL over the werewolf tobe honest, as a VL you get the rested bonus as well as some realy nice powers.

I think Serana can turn you into a vampire whenever you ask her to after you've completed the Dawnguard questline. I'm not entirely sure though.

I am pretty sure that you can turn back though. Just not sure how.

I found this on wikia: You will not lose any Vampire Lord perks if you cure yourself of vampirism and then ask Serana to restore it.

So apparently yes, it's possible and on top of that whatever progress you've made as a Vampire Lord will remain with you

Ah ok thank you. I just kind of wish there was an option to refuse them but instead tell them you will help them anyway.

Yeah, it would be nice if you had the option of chossing whether you want to become a werewolf or not. Just make sure on the last quest you cure yourself then, because you have to do a bunch of radiant quests before you have the ability again. I'm still stuck with lycanthropy because they won't give me the correct quest necessary to cure yourself.