Without trying it and completely off the top of my head, I would suggest that the RetroPie image is as lean as possible and VNC simply isn't installed / enabled as this is not how most people would be using it. I would be surprised if RetroPie had any desktop support at all but I could be wrong.
I would suggest making up another microSD card with RetroPie so you don't ruin your known good card and follow some instructions on installing VNC, eg the ones here:
https://www.raspberrypi.org/documentati ... ccess/vnc/
... and see if it works before trying it on your good card.
I am doubtful that this would work as RetroPie is probably configured to only use the physical display hardware (HDMI, Composite) and would not render anything to a desktop. But as I say, never tried it and happy to be proven wrong.