There are several settings that affect this. One is power management, one is the screensaver and one is the X system itself. I found that I need to run
/usr/bin/xset -dpms s off s noblank s 0 0 s noexpose
every time I start X. In Ubuntu 20.04 I did this by adding a custom startup application in my user settings.
Yes that works. Have a look at the power management too. And keep your VNC session running for half an hour. If the screen still blanks then you can try to to run the xset command and see if the screen stops blanking half an hour later. If yes then you can add the xset command to your start up applications.