I heard Vingalmo offers the quest of turning your spouse into a vampire - quest called "The Gift" according to most searches. I have completed the vampires quests, in fact I've done all the DG quests by now afaik, and I've completed Vingalmo's other three quests, as well as dozens and dozens of radiant quests from the other Voikohar castle NPC's.

Having done countless of others of the radiant quests (for instance, I'm fresh out of "important members of society" in Windhelm and Riften now...) every time I get referred to Vingalmo I end up with another quest to turn either Harun or Valindor into vampires. I already turned them both, and I also turned the girl in Markath, but killed her when I got the quest from Vingalmo to turn her again. After that I made sure to save my game just before turning in a radiant vampire quest. Now, the only quests I keep getting from Vingalmo are quests to turn Valindor and Harun into vampires. I can't get "The Gift". I've been married to that Breton girl in Markath's alchemy shop, since before downloading Dawnguard and she lives in my house in Solitude.

Now, I know of the bug about Vingalmo offering the same quests again and again, but I don't understand why he then doesn't even offer "The Gift" once. I've reloaded games hundreds of times in a row after quest-turn-in to see if Vingalmo offers "The Gift", but he never does.

So, are there any prerequisites I've neglected? Should I not have killed that girl in Markath who he wanted to turn into a vampire (again)? Are there any other spouses than the Companion ones who make the quest impossible to get? Or does the quest to turn your spouse into a vampire simply not exist for the PC version of Dawnguard?

These quests come randomly. I have played through tons of side quests and never received any of the quests to get the Dwemer blue prints (I believe that is what the vamps area asking for), It is random and besides being married I have not found any other prerequisites.

Its nothing you did. The quest is broken as heck for some. I have never gotten it either. It varies from game to game but there is a good chance he will just never give it to you.