If i become a vampire at the half point in the dawnguard questline. can i go baqck to the dawnguard after finding Serana's mother?

No. Isran will tell you to get cured. Just wait til questline is over, then turn into a vampire lord. Serana will give you the option after Harkon is dead.

I thought you could become a VL to enter the soul cairn then cure yourself and then move on with the Dawnguard quests. Many people have posted this. I chose the vampire side so I do not have first hand knowledge.

Yea, you can. I think the op meant can you proceed as a vampire through the dawnguard questline.

You don't need to be a VL to be able to enter Soul Cairn. Serena can cast a "partial soul trap" spell on you instead, and once you found her mother in the Soul Cairn, you can/should retrieve your soul in a chest inside the Soul Cairn (marked with a quest arrow). But really, with Falion in Morthal you can cure yourself at any time, and unless you have turned Serena human after helping the dawnguard vampire hunters, you should be able to turn into a VL at any time. Though Serena's mother might also be able to turn you.

He's not saying that, he's just saying if that is an option.

sorry i was unclear, im past soul cairn now. but i was refering to it i couldnt make up my mind which way to go. i was attempted to go VL. but i was really not wanting to have to go morthal to cure myself