Where in the game can the Robe of Vecna be found.
I know where it is in Shadow Keeper.

The bonus merchant in Ribald's Adventuremart sells it. She is standing in the middle of the screen if you walk past Ribald ino the upper area.

As JSBB pointed out, it's a bonus merchant. But to get her, you have to make sure that you have that as part of your version, or have downloaded them.