How do you become a VL and do the dawnguard questline

You have to complete the Dawnguard quests and then ask Serana to turn you afterwards. Although if you do choose to become a Vampire Lord the Dawnguard will refuse to speak to you until you get cured.

Thanks

Also, at the Quest where you enter the Soul Cairne you gain the opportunity to become one.

I did not know that