Finding Vados

It begins when you reach Desert Prison Cells, once you've completed Destroy Vados.

It lists no rewards of its own, so finishing it pays nothing but its reputation.

It's one of four quests, with Playing with Salem, The Mad Scientist and Deena Is Reborn, that must all be complete, after a certain point in the story, before the game counts you as ready for the Abyssal realm.

How quests move on, pay out and count toward your reputation is on the Quests page.