This would be a fantasy setting where humanity figured out that vampires existed. Terrified of anything different, they started a campaign to destroy the problem. Most vampires were simply killed, but a few were kept alive in captivity. Their captivity was hardly public knowledge, but a few discovered that the life essence of a vampire is powerful and recharges fairly quickly. A mage, even a fairly unskilled one, can tap into that life energy to cast more powerful spells.
Ethan West is a novice mage, sent by his instructor to wander the land to gain firsthand experience. Mostly he helps areas where crops aren't growing and assists villages suffering from bandits and minor monsters. After preforming a few tasks for a small village, the village elders are forced to admit that they don't have the coin to pay him. Instead, they offer Ethan a vampire slave. She belonged to the village mage, but he passed on, and the town has no need of her. She has been a captive for hundreds of years, denied human blood to keep her weak and trained to submit to the wishes of a human.
Ethan has no desire to harm the woman, and his kindness starts to kindle affection between the two of them. But a real relationship could mean peril for them both. Even feeding a vampire human blood is illegal, so loving one would get them both killed if others found out about it. Still, as time goes on, the problem becomes more and more real.
I imagine this would be a dom/sub story with a kindling romance and a number of supernatural elements.