As Ondra posted (while I was typing) basically it´s a one-liner:
CoronaRenderer.CoronaFp.showVfb(true)
To very quickly create a button:
1. Open MaxScript listener
2. Paste that line
3. Mark the line in the listener window
4. Drag to a Toolbar, done.
But also have a look at this:
https://forum.corona-renderer.com/index.php/topic,8930.0.htmlThe show vfb button in this toolbar does further checks (also checks for current renderer and some more)
Good Luck!