Major Vampires can continue to drain people regardless of distance or walls.

[Lady Birb]

Someone was being drained by a vampire so I dragged them away and shut the door, but the vampire continued to drain them despite the distance and barriers between them and their victim.

[mbc]

vampire Drain is a ranged spell that is less efficient and takes more time than a Bite. its also intentional that it goes thru walls. You can stop the drain by leaving their range or forcing them to move one tile