Vampires don't really use heavy armour and they don't want to cover up their beautiful faces
Also they don't need weapons (harkon's sword is the only vampire weapon you need)
They don't need weapons? Well maybe the npcs don't because their spells are actually good, but the player vamp needs weapons since your spells are useless. Also, I think they should have added heavy armor. Not everybody who becomes a vamp wears light armor (I wear heavy armor exclusively).
Bethesda really gave vampires the shaft in Dawnguard, which is strange considering how it's a dlc about vampires, and it was stated they would get an overhaul. Looks like more http://www.youtube.com/watch?v=6ne1fVhvL00.