I had a similar looking rear bumper when I bought my VX. I pulled the bumper off, and heatgun and propane torched the backside (at a safe distance to not melt the cladding) to get it soft enough. Also making sure to focus on the creased areas so it doesn't tear, took an old partially inflated basketball, stuck it to the back and pressed the dent back out. I believe I did it 2 or 3 times before it came out all the way.

Now doing it that way, I can almost guarantee that you will still have crease and stress marks left over.