@Hank That's where your use of Mauloch comes in. Seeing as there isnt going to be a dark elf during our RP thats a PC I won't go into far enough detail.
The orcs of the nation of Orsinium do still worship Malacath in a way, but in Orsinium it's spun as Mauloch, the lost elven warrior god Trinimac who was turned into Malacath by the Daedric prince Boethia.
Mauloch has non-deadric connotations in the form of a reborn Trinimac from the Altmeri Pantheon. Whom his followers were twisted and turned into the Orsimer as punishment for wanting to stop the then (Chimer, the dark elves before they were dark elves) from leaving the summerset isles in exodus into Tamriel.
The worship of Trinimac was revived by Gortwog gro-Nagorm, the ruler of the Third Orsinium. While many of those living in the city agreed with his beliefs, the Orcs outside still devoutly worshipped Malacath. (Mind you this is before it was destroyed during that era, but it still qualifies.) The orcs when brought together as a whole in Orsinium. Have always worshiped Trinimac as the true patron of the Orsimer.
"Trinimac the Warrior, the Paragon. Once he was our patron, our god. He encompasses the true ideals of the Orsimer―strength, honor, and unity."
Again, meaning why he would not just go out and the vigil would just raid orcs? Because not enough Vigil, but they would not attack Orsinium because they revered Trinimac a Non-Daedric entity. Same would go for Morrowind, as one knightly order can't stop an entire Provence of Daedra worship. But still able to enforce the secret worship of the more terrible ones (Merunes Dagon, Mephala, Sheogorath, Clavicus Vile, Hircine, Molag Bal, Boethiah, Namira, Nocturnal, Peryite, Vaermina) As well as Vampirism, Lycanthropy, and Necromancy.
Can't ever stop full Daedric worship, but you can put the fear into any that do secretly that any day the Vigil will come to their door. Would stop the whole world ending terror seen Via Oblivion Crisis.