there's no menu option to minimize from full screen. To go from full screen VNC viewer to another app you have to switch out of full screen and minimize, this is a time waster.
You can use applications on your secondary screen while tigervnc is displayed on your primary screen (or the other way around).
If you set up tigervnc not to pass special keys to the remote session then Alt+Tab or Win shortcuts should work even in fullscreen to switch to other applications and windows menu, respectively.
If you set up tigervnc to pass special keys to remote session then it does so, including Alt+Tab, etc.
Note that you can switch applications normally while Tigervnc is not fullscreen, you do not have to minimize.