I just finished the quest where you kill Harkon. Now that thats dealt with I really wanna join Dawnguard and tell the other vampires to f off. Is this possible or will they be hostile to me forever? Oh this is probably obvious but I sided with the Vampires when doing the main quest.

The Dawnguard will be hostile to you forever. If you want to join the Dawnguard you must do so when given the option early in the quest line.

Spoilers....and no. I believe you can't.

Yeah sorry about the spoilers I forgot

No...however, you can cast calm on them and use them as vendors. You can even buy a troll (although I can't see why one would want to). Either way, it's an awesome way to collect new spells and such. Either way, if you're a vampire, you pretty much have to join Volkihar.

Why would a faction dedicated to the extermination of vampires, allow a Vampire Lord no less join them?

Yes, please explain why they let Serana in...

I didn't know this thanks for the info

But be careful. They tend to be a pretty high level.

Because you can vouch for her and she brings the Elder Scroll with her. Might be that helps.

And Serana cant vouch for you then?
Surely not all vampires want to see the end of the world as we know it.
My vampire is a 'good guy', never feeds on anyone but bandits, helps people etc.

Serana herself is also a vampire(in case you didnt notice:p) a vampire vouching for another vampire before the dawnguard wont stop the dawnguard from killing you. As for being a good vampire i dont think isron cares about that, he really hates vampires and wants them all dead, he wont give you a chance to proof yourself as a vampire. Which makes sense tbh.

But I don't think that's the point Merari was making.

Saying that Isran can't abide having Vampires in the Dawnguard Fortress to explain why he won't allow the player who wants to play a good vampire in, shows up that the Dawnguard MQ has some horrible lapses in logic and plot flaws.
For example -
Say I've been RPing a 'good' Vampire for most of my game, I get the Dawnguard DLC and off I go and join them. It all goes fine until after I bring in a couple of new recruits and Isran finally realises I am a vampire. He sends me off to get a cure. (If I wasn't a Vampire at this point Isran and I would then immediately go talk to Serana who has shown up in the castle with the Elder Scroll. I'd vouch for her being helpful to our cause, and that would be cool with Isran.) So I go get a cure, come back and then get to vouch for Serana who's shown up. From that point Isran basically allows her the run of the castle unsupervised. At no point does he demand that she get a cure. I go off and get re-vampirised, but Isran will not allow me beyond the foyer of the castle and demands I get cured again. While I'm standing there Serana is wandering around the castle. Yes, Serana is allowed in because I vouched for her being a good vampire, but I cannot vouch for myself being a good vampire.

So I was a Vampire when I joined the Dawnguard. I vouched for Serana. It's cool with Isran that she's a Vampire, but it's not cool with him that the person who vouched for her is a Vampire. Make any sense?

ha isran does, he only allows it because they need her and her elder scroll to stop harkon...why he lets her stay after that is beyond me, maybe she nibbled his neck 1 night and he liked it

Fair enough, but isran states she is not welcome she is a asset. but isran doesnt have a choice they need her help(she has the elder scroll and knows where to find the second). But if you become a vampire your pretty much useless to the dawnguard. And how would it look to your recruits and the rest of skyrim, calling yourself vampire hunters but you allow vampires in your ranks?. So much for your credit in the rest of skyrim. Also isran doesnt completly trust you to begin with, i mean you came across a vampire. She could easily have charmed you while you travelled with her. And becoming a vampire migth have crossed the line.