The Gift

Walkthrough
After playing a few Radiant quests for the vampires at Castle Volkihar, Vingalmo will quite bluntly point out that it would be oh such a terrible tragedy if something happened to the Dragonborn's spouse. (Obviously the quest only comes up if the Dragonborn is actually married.)

To better protect the spouse, he therefore suggests turning her/him into a vampire.

Mesmerize and turn  into a vampire
Use Vampire Seduction on your spouse and then bite her/him, bestowing Lord Harkon's gift.

Return to Vingalmo
Returning to Vingalmo may yield an item like:
 * Circlet of Conjuration