I married Muiri and I've completed the Volkihar questline and I'm trying to get "The gift" quest from Vingalmo.

I've already done about 6-8 quest for the npcs around the castle but Vingalmo still hasn't given me the quest.

It took me awhile to get it. I wanna say close to 20 side quests. At least that what it seems like.

FYI, I married Muiri as well.

Does the gift quest trigger if you marry someone from dragonborn?

It triggers as long as you have a spouse.

Oh ok thanks i didn't know if the characters that are marriable in dragonborn, were also turnable. thanks.

Not for me it doesnt.

On every playthrough since the release of Dawnguard I just get sent to turn an NPC I already made a vampire, breaking the questlines.

Reloading does not fix it.

Thats happened to me as well. he unfortunate thing is I had to take those NPC's out, for the sake of keepign the questline going.