You don't need a vapor barrier under the pan, but you might want to put one to keep the mortar from being sucked dry by the plywood until it cures. You should also have a vapor barrier behind the cement board - bring it down the wall and lap it over the flange into the pan.
To keep the fiberglass from getting stress fractures and to make it feel more substantial, it isn't a bad idea to have enough down there to essentially cover the whole bottom, or at least the majority of it. It needs to be deep enough to allow the cavities to be filled in. Just make sure to smush the pan down until it is level - that is critical for proper operation.