Where are all the random dungeons?
I've never seen this and I've been through Act IV many times.
I realize there is a WIP list of where all the random dungeons appear, but this is tailored for Sean's question specifically.
You can find Sacellum of Virtue, a randomly generated level, on Gardens of Hope Tier 2.
So the usual applies. If, after you've explored the whole floor and it's not there, then quit the current game, resume it, then use the namesake waypoint to teleport to the level.