I am working on a similar problem and I having some issues with the Epic VRandCone function. Despite the comments and what the algorithm should be doing I am still seeing clumping/hotspot in the middle around the cone direction vector when I fire my rays. I tend to end up with very few on the outside extents of my cone. Given enough time/samples it starts to even out more but for my current setup it is a bit of a problem as I need a better distribution. I am starting to wonder if it is more of just a geometry problem so that even though the ray distribution is uniform I will still be getting something happening around the cone axis due to shallower angles?
I do know the plane these rays intersect on and could do a sort of rejection sampling and refetch a new random if there are too many rays per area on the intersection plane. Sorry for the rambling but I am a bit lost with this function.
Any thoughts?