That's always depends on difficult and play-style.
The Royal-Vampire-Armor gives 125% magica regeneration. Since I'm using in the end Dremora-Lords as tanks, that the important number for me, not the armor-rate.
The dawnguard-quest-line always starts with you giving a crossbow, letting you find a lot of mage-robes, worn by vigilants, and ends with your shooting at Lord Harkon with Auriels Bow.
One of the first things to learn as a member of Harkon's court is: That raises the chance of being attacked by a vampire!
That means, most advises and strategies for a vampire-hunter are also good for the hunting vampire.
Like the spiders mentioned by Bonedog1!
With a health-skill of 40, you can get the perk for restoring stamina with health-spells. With a one-handed weapon and a nearly infinitive amount of stamina thanks to spells in the left hand, the armor-rating becomes secondary again... ... like with the power-shot-perk in archery.
I did not yet use the death-hounds as pets on PC. On the PS3, they did resurrect themselves, except when Serana did use her necromancy-spell on them. So, they where perfect tanks for archers, until Serana let them turn to dust.