It's been a bit of time since this post, but I wanted to ask for some additional details on how you implemented this using VEX. I'm a beginner at Houdini so this is proving a bit difficult for me but as you say ,I'm fairly certain this is simple and I'm just not looking at it correctly.
I don't understand what you mean by “refracting the point cloud through the noise deformed grid”, by that do you mean you use the VEX refract operation? And does that mean you set the point cloud positions to the end result of the refract?
I think the rest I understand, where you'd take the results of the projection and use a Ray SOP to project onto a flat grid plane for the final caustics.