Although any class could play and enjoy the vampire questline, by sheer luck I happened to dive in with a level 14 Breton conjurer/necromancer without really knowing much about the story. I have really been enjoying the fact that a lot of this questline seems particularly suited to that "class", with all the spells and some of the gear you find. The fact that I'm playing as a conjurer has allowed a greal level of immersion in the story, with so much of it tied naturally to those skills.

Out of curiosity, is there any particular class or build that is a good fit for the Dawnguard side? I assume a lot of it is warrior-centric. Are there any good skill trees to have leveled, such as archery or smithing?

Archery for the Dawnguard, mainly because they use crossbows.

I have a heavy defensive Paladin type character that uses heavy armor as well as a shield and sword combo, the dwarven crossbow design definitely suits The theme of my big bulky tank character than some sort of precise bow assassin.

My first playthrough was with a stealth/archery assassin character and I had a blast. I'm currently building a tank/two-handed Orc with an emphasis on Restoration spells - cuz there's some great ones used by DG. Honestly I think most styles will work though some have criticized that there aren't as many Mage options for DG. Battlemage, yes, full (robed) Mage not so much.

My level 12 Orc is a 2H, Heavy Armor specialist and she seems to fit in very well with the DG. I'd agree that a full robed mage doesn't fit so much.