All vendors get rot. But it actually only applies as long as you are near them - when you are away, rot stops at the level reached in the last interaction.
This rot applies even to the undead trader in hunters Edge, which is odd, as it is counterinzuitive to the whole story about undeads in the Luculla mines.
The only vendors exempted are all elementals and traders in homestead, including the intergalactic one (logical, he has Tenebrium stuff on offer). And the Tenebrium traders in Sacred Stone (again logical).
Regards,
Thorsten