To clarify, which of these sequences is/are true?
Become vampire, then take Steady Hand 2/2 = bugged
Become vampire, then take Necromage, then take Steady Hand 2/2 = bugged
Take Necromage, then take Steady Hand 2/2 = bugged
Take Necromage, then become vampire, then take Steady Hand 2/2 = bugged
2 and 4 are true. It does not matter whether you become a vampire before taking Necromage, or vice-versa. It's the things you take once you have both vampirism and Necromage that get boosts. Like someone said above, these effects remain boosted even if you cure vampirism, so long as you don't lose the effect. For example, you can take the Atronach perk and activate the Atronach Stone while having both vampirism and Necromage for 100% Spell Absorption, and if you cure yourself you will keep that 100% spell Absorption forever, so long as you don't pick a different Standing Stone or reset your Alteration perks.
So if you want something to be affected, get it after you have both Necromage and vampirism. If you don't want it affected, get it before you have both (having just one or the other is fine).