So I investigated the cavern and found the young vamp chic with the Elder Scroll. Porblem is, I don`t want to become a vamp. Will I still be able to play both the vamp and dawnguard questlines if I refuse or accept being a vamp off the bat?

You saw and talked to Serana and weren't corrupted immediatly by her?

If you don't accept the Vampirism when offered you are banished from Castle Volkihar and as such can't do any quests for them.

Oh and truly a virtuous soul.

It's one or the other, sadly. You refuse Harkon's offer if you want to be Dawnguard, and accept it for VL.

You cannot accept vampire and go directly to the Dawnguard, but since you don't want to be a vampire anyway, just refuse and go to the Dawnguard like you planned.

That svcks. So its one or the other I guess.

Actually i heard that you can become a vampire lord even if you do dawnguard side quests. I heard that near the end of the dawnguard story line, that vampire chick can make you a vampire lord.

It's true sadly, but the OP asks about the beginning.

O, sorry then.

Spoiler box man, this is the general discussion section.

Good !