A mod for vampirism is a must if you plan on doing the Volkihar Vampire Clan quests and not siding with the Dawnguard. I recommend Better vampires by Brehanin: http://skyrim.nexusmods.com/mods/9717
This mod completely overhauls vampirism and makes it much more interesting as well as useful. It gives you heaps more spells which you learn over time as you feed, a counter for feeding which slowly makes you stronger as you feed on more people in the form of buffs to your skills in combat, sneakiness and magic. Also feeding on people will damage them, so you cannot just sit there feeding on them infinitely. After two feeds they die. You may also feed once on recently killed corpses and drain them of blood. You can also set it up how you like to a fair extent, it gives you quite a number of options. For instance you can choose if your vampire is most powerful if it has just fed or if it has gone some time without feeding. Likewise You can choose how your powers are effected by sunlight, how much sun damage, if any, you wish to receive and when you receive it (always, only when starving etc). You can of course change how its set up at any time by using the power that comes with it to change any settings. I find this mod makes vampirism a much more immersive experience. There's a couple of vampire overhauls that I have tested out, but the customisability of this mod makes it my favourite.
Also if you wish to become a vampire lord, I'd recommend a mod that lets you loot and activate. It doesn't make sense that powerful vampire lord forgets how to use a door knob. It makes becoming a vampire lord much more viable. This mod: http://skyrim.nexusmods.com/mods/21723
does this as well as buffs out the vampire lord form considerably. Use this only if you're a high level character otherwise your vampire lord will be extremely over powered, otherwise use this one http://skyrim.nexusmods.com/mods/21631
That's the major Dawnguard mods I'm using. I'm obviously a vampire, so those make significant changes to how I play.