Is there a switch to avoid that the screensaver is activated on macos when VSTL is running? I only found an old feature request in the forum.
And can I run VSTL in fullscreen (not only maximised)?
I feel kind of dump asking these questions 
but I don‘t find any switches.
At any computer you use live, better to turn off screensaver (and all of such options). I personally on Mac I use - CMD+ALT+Eject(/or PWR) - buttons to turnoff+lock screen. Or using Hot corners for Screen off.
Both are better options then letting screensaver auto-activated.