SPOILER!!!!!!!!!!!

Just wondering because I don't have DG yet but I'm thinking about making my character into a (vanilla) vampire and I heard that the dawnguard won't except you if your a vamp... Yet I know that they send you on a quest which introduces to the volkihar vamps....... Sorry if all that sounded jumbled together and thanks in advance

Yes.
Worked fine for my character from before the release of Dawnguard.

You can simply get the quest from Isran. Get infected. Turn into a vampire, and then resume the quest. Everybody (including Serana) will recognise you as a wild vampire in Castle Volhakir.

Dumb Dawnguard. I like them, but they aren't much logical sometimes. I mean, you can't progress as a vampire in the DLC story but you can actually start it as one.

There are mods to bypass the vampire checks to complete the whole Dawnguard DLC as any side(If you're playing it on the PC)