so, does anyone know how to change these, if you want to play a quick round of vql race for example?

now it would start with pql physics, and nothing else you can set there, when you start a match.

BTW.:
custom maps are downloaded to ...steam\steamapps\workshop\content\"xxxxxxxxxx"\"xxxxxxxxxx"

to play them offline with "Start Match", you have to copy the included *.pk3 files to
...steam\steamapps\common\quake live\baseq3