A School of Necromancy Wizard's Command Undead ability allows them to control an Undead creature if that creature fails a CHA saving throw, depending on the intelligence of the creature they get benefits to the save, INT 8 or higher gets advantage on the save, 12 or higher gets a save every hour.
The ability can affect Vampires but they would fall into the group that gets a saving throw every hour since they have an INT of 17, if I wanted to keep them under my control permanently I'd need to make them more dumb.
The Feeblemind spell can set their INT to 1 but they can possibly regain their INT every 30 days. The Idiot card of a Deck of Many Things can reduce it by 1d4+1 permanently but would require 2 decks, convincing the Vampire to draw cards from it, and enough luck to draw the Idiot twice.
So what ways are there to reduce a monster's INT score? Is there a different monster that I could bargain with or control through other means that could reduce the Vampire's INT?