You can become a vampire any time you wish by fighting vampires and letting them hit you with their "Vampiric drain" spell until you get a notice informing you that you have been infected with "Sanguinaire Vampiris" the vampire disease of Skyrim. I have only played through the vampires side of the story so far, and, since my Character was already a vampire, it seemed like a good idea to check out the vampire lord form and the powers that come with it. While it's not quite what i expected, it's still pretty cool and some of the abilities you gain are quite helpful. I suppose which side you choose is up to you and your own personal preferences, but joining the vamps is a good place to start the dlc in my opinion. As far as pros and cons goes.... The pros about joining the vampires are: the vampire lord form, immunity to diseases and poisons, death hound companion, a new story to experience and dungeons to explore. And cons: The vampire lord form starts of relatively weak and is quite vulnerable, you level up the vampire lord skill tree by using your drain life power in vampire lord form and power attack biting enemies in melee combat in vampire lord form, each new level takes more killings. It's been quite a lot of work to level up the vampire lord form as much as i have but i think it will be worth it in the end. Have fun and good luck with whatever side you choose.
