So I was poking around the creation kit and console commands with Dawnguard and I stumbled upon some interesting things.

There are unused dialogue choices in the dawnguard questline. Nothing fancy. There is a dialogue choice that leads me to think that the dawnguard questline could be started just by going to dimhallow and finidng Serana without any alliagence to the Dawnguard as "SeranaMeetsTolan" Answering Serana "Who sent you here? - No one " and Harkon's trailer dialogue 'What is it that you seek..." . I think that they are unused. Maybe I am missing something. Is there any other way to start the questline without going to castle Dawnguard? I found that the Hall of the Vigilant is destroyed after the questline starts. It's all well and good until you stumble on Durak or a guard talking about the Dawnguard, after the dialogue pops up, the Hall is destroyed. And there is no way to pass the first gates in Dimhallow until you talk with Isran. If you went there before talking with him, there is a big rock in front of the gates, and the cave is filled with wolfs. Also You can take Serana to Castle Dawnguard but she will stay outside of the valley. You can report to Isran about Serana that she is a vampire and has the scroll, and he himself will order you to take her to Castle Volkahir. Also you can not save vigillant Tolan the moment he zones from the Valley he is teleported to Dimhallow and killed by vampires.

Maybe a kind modder could use this information to add some new ways to experience dawnguard . The dialogue choices and voice files are in the game .

Is the Hall of the Vigilant completely destroyed ? I liked visiting it from time to time and seeing all the Vigilants, for the lore

It is pretty much burned to the ground.