It just doesn't make any sense.
Who cares if the Dawnguard accept you as a VL after you've finished the questline? I'd be more upset if they didn't.
Hell, Serana, the Vampire daughter of the antagonist, has just helped a group of Vampire slayers kill her own dad and save the world from an affect that could only have benifited her. If that doesn't prove to the Dawnguard that all Vampires aren't saddistic [censored]s, and that some can be genuinely good people, nothing will. If your character wants to become a VL after saving the world, then use those powers to hunt other Vampires, there's no reason for Isran to care.
I agree. There are lots of reasons why it would be possible to become a vampire after the DG questline for the story, Serana, etc. Though since the DG will always dislike you if you are a VL, then I'd say it fits the story well as it is. I mean, you can have Serana cure herself, so why shouldn't she be able to make you like her, if you accept it?
I have to agree here to it makes no sense. And if this is true then I should be able to get all the new spells from just doing the vampire side.
You can. Go to Fort Dawnguard and use a Calm spell to buy the spells from the merchant there. Easy, and it fits lore, as vampires can hide and use charm and magic to manipulate people, even their enemies. I scouted out the Dawnguard and found their weaknesses as a regular vampire when I started the DLC, so I could know more about my enemy before I gained power from Harkon so I could become the master of the Volkihar.
Also, why are people complaining about this? You suffer consequences according to whether or not you want to be a werewolf, VL, or a mortal....so....what's the issue with being able to change your mind? I figure everyone would have more of a problem with being able to change back and forth between the 3 with such freedom and without much difficulty or permanency. Now THAT I could see being a gameplay choice BGS made for DG that disrupts proper lore more than anything else. But, whatever....