for the dawngaurd dlc im going to join the vampire faction so my question is should i become a vampire before dawngaurd is released or wait and become a vampire through the dawngaurd questline? let me know what you all think

We have no clue how pre-vampires are going to be treated.

Or werewolves for that matter.

I'm guessing they'll have an acknowledgement, sort of like how the "Vampire" quests in Fallout 3 had an acknowledgment if the player already had the Cannibal perk.

I would, just so you can get a feel for how the vampire plays in skyrim as of now. Many things regarding their abilities and behaviors to certain things (such as the sun, time with the disease, etc.) will most likely be changed throughout the expansion, but trying one out beforehand would probably best prepare you. And if some issue arrives, the beta testers will more then likely contact and inform Bethesda to make a necessary change before the add-on is released.