Lord Harkon gives you two options:
I will accept your gift, and become a vampire
I do not want to become a vampire, I refuse your gift
If you wish to side with the Dawnguard, and be a Vampire Hunter rather than a Vampire Lord, choose the second option and REFUSE his gift.
Lord Harkon tells you that you are prey, like all mortals. He banishes you. He casts some spell on you, and you find yourself outside Castle Volkihar. Open your map, fast travel to Fort Dawnguard. The fort is under attack by Master Vampires, Vampire Mistwalkers, and Ancient Vampires. Help the Dawnguards kill them.
Speak with Isran. Tell him what happened, that the Vampires were looking for a woman trapped in Dimhollow. Isran wants to know who she is, and where she is.
Your dialog choices are:
She wanted to go home, so I took her to her castle
She's the daughter of a powerful vampire lord
Choose the first option. Isran is angry at you, for not stopping them and not securing the Elder Scroll. You can say:
I'm lucky I made it out alive
There were too many of them, and only one of me
I never got the chance
Either way, Isran is upset. The vampires have everything they wanted, while the Dawnguards are left with nothing. He thinks this couldn't get much worse, and is more than you and him can handle. If you ask him if he is giving up, he says he just needs help. If the vampires are bold enough to attack the Dawnguards in their own fort, then this may be bigger than he thought. Isran tells you that there are people he has met and worked with over the years. We need their skills, their talents, if we are going to survive this. If you can find them, we might have a chance.
Ask him where you can find the people we need. He tells you that we will want Sorine Jurard. Breton girl, whip-smart and good with tinkering. Fascination with the Dwemer. Weapons in particular. Last Isran knew, she was in the Reach, convinced she was about to find the biggest dwarven ruins yet. You'll also want to find Gunmar. Big brute of a Nord, hates vampires almost as much as Isran does. Got it into his head years back that his experience with animals would help. Trolls in particular, from what Isran has heard. Gunar is out scouring Skyrim for more beasts to tame.
Isran tells you that if you bring the two of them back here, we can get started on coming up with a plan.
Check your quest log:
A New Order
I've told Isran what I learned at the vampire castle. Isran has asked me to find Sorine Jurand and Gunmar, and bring them to him.
Your new objectives are:
Recruit Sorine Jurard
Isran is a new heavy armor trainer, so talk to him if you need training. If you ask Isran about Fort Dawnguard, he tells you that this fort was originally owned by the Jarl of Riften back in the Second Era, and that it was a bit of a black mark on the Hold's history. Isran is trying to put the fort to good use. You can also ask him for advice for a new vampire hunter. He tells you that when it comes to vampires, if you're sloppy or careless, you're dead,. And good people will die because of you. He advises you not to take chances, cover your tracks, and keep your eyes open.