Looking for confirmation

Get perks that are affected by necromage before you become a vampire is the only way to get necromage benefit? What happens if you get cured and then go back to being a vampire?

Using pacify on dawnguard to get stendarrs aura? Ive heard its possible. Assuming tou have to sneak in or do you try and get cluster under spell
? Do you need that quest assignment in order to even try this?

I don't have Dawnguard, but as far a necromage goes you want the exact opposite. You want to be a vamp. Take the necro perk. Then take the other perks that are affected by the necro-vamp combo. You can then cure vamp if you like and the boosted perks will still be in effect. Even gear equipped during necro-vamp will remain boosted as long as you don't ever unequip them.