After using vde2 enabled qemu on Linux first time this night I wondered if it was ported to NetBSD. Apparently no, not really yet, while patch from year 2015 exist:
https://github.com/PDP-10/klh10/issues/2
For single qemu you surely can just use built-in slirp networking, but for network of them vde2 seems to be better fit.
I'll poke this patch, but I am nowhere near even admin level, let alone network-related developer, so if it failed to run with cryptic for me err msg I most likely will give up :(