I've made a thread about it somewhere in July. But I want to go into details with this, because I'm really loving the idea.
However, what makes it difficult for me is the fact that a vampire seems to be bound to Molag Bal, while a Nightingale is loyal to Nocturnal.
Obviously, one can't go to both realms upon death, so how could I justify this? Perhaps she won't even die in the first place, being a vampire at all. But still, you don't want to end up angering Daedric lords.
Anyway, this character will be a more darker version of Robin Hood (Not the skills, more like the motives.), a Dark Elf that works with the Thieves Guild to steal from the Nobles, and to give to the poor.
She also works as a maid for the Jarl of Riften, as a disguise. But at the same time, I also want her to be a vampire, as it seems to fit the shady young maid.
I think I'll do the Thieves Guild somewhere at lvl 20, to atleast have decent leveled equipment. No assasin stuff though.
But to the main question, as the title suggests, how could you possibly justify a Vampiric Nightingale? I considered that she would become a Vampire after doing the TG questline, because she liked the idea of everlasting youth and the vampiric abilities.