In Dawnguard, can you join both the Vampires and the Dawnguard?
Obviously not at the same time (though tricking them into accepting a Vampire Lord and having an option to betray them and kill everyone for the Vampires would have been a cool option), since that would not make much sense in some regards.
But what I mean is that, if you are cured from being a Vampire Lord, after you do the main quest for it and some of the side quests (like the awesome rings being a Vampire can give you. Well, mainly the Ring of Erudite, with how much Magicka it can give no matter how far you are in the game, and have your magicka recovery rate increase... by how much, I have no idea, but it helps!

The main reason I would want to is because... well, I prefer to travel during the day, and as a Vampire, doing that will not let your stanima and health recover by itself (which gets really annoying when you have to run to a far off place, but I suppose that's the point, making it better to travel at night. And what's with the Vampire Sight? I hate how you have to resue it every minute and not be able to keep it on... but that's just me getting off topic). I would also love to be able to get the stronger version of the Crossbows, and be able to make my own Bolts so I don't have to worry in the back of my mind if I am going to run out or not. I know I won't, but the worry is still there.
So... yeah... Can you be cured with being a Vampire after doing the Vampire's quests and join Dawnguard and get their Crossbows?