Naginata and Tyer said practically all .... but beside that, for me it does not make any sense to become a vamp. Our character, the dragonborn, was setup in the game to be a savior not a slayer, a protector not a people’s enemy. He must save Skyrim from Alduin, Ancano, Miraak and the vamps. When you download Dawnguard, every single guard will say to you : “they are reorganizing the Dawnguard. These vamps are getting a real menace. I am planning to join myself ...” something like that .... so what does that means ? Simply that vamps are a menace; they kill innocent people. When they enter cities, they are killed by whoever : guards, citizens, etc ... all of them enter in battle against them. Guards do not tell you : “vamps are getting stronger. I should become one of them, etc, etc “ Why ? Simply because vamps are enemies as Alduin, Miraak and Ancano.
Coming from this standpoint, for me, does not make any sense to become a vamp after I practically saved Skyrim from Alduin, Miraak and Ancano. Now that you are a “good guy”, a hero, a savior, is it right to turn to the dark side and become a menace ? I don’t think so !! Doing so just goes against your own nature and the reason that you exist.
After you kill the freaking Harkon, you can turn Serana into a real human and she does accept, that means that she also regret that once she was a vamp. And, if you become a werewolf thru the Companion quests, is just to protect yourself from getting vampirism and once you are done with Harkon, then you can cure from being a werewolf and become fully human again. You do not have to play in the “good side” but doing the contrary in my opinion, contradict the whole scenario and the reason for the dragonborn to exist. Just my 2 cents ...