problem
Seems like PBR is brittle or broken. Creating a simple source route wipes out the default route, and system is offline after that. I dont know if that is expected on this platform but it is unexpected to me.

config
set interfaces ethernet eth0 address dhcp
// commit was done here, system was online / could ping out

set interfaces dummy dum0 address 192.168.188.1/24
set protocol static table 200 route 0.0.0.0/0 next-hop 200.252.252.252
set policy route foo rule 400 source address 192.168.188.0/24
set policy route foo rule 400 set table 200
// commit was done here, get warning, and unable to ping out anymore

console output
Warning: priority inversion [policy route foo](201) <= [policy](470)
changing [policy route foo] to (471)

other stuff
policy inversion appears to happen when using other selectors as well as source address.

vyos environment was live iso on a kvm instance

got the VERSION_ID from /etc/*release (1.4-rolling-202104202252)