Try changing the kernel name in the mounted root hierarchy from netbsd to 
be netbsd.vax
if I'm right this will allow you to boot. There is an annoying bug that 
I've tickled a couple of times but not swatted where the second path 
through the boot code isn't "clean" from a networking perspective. By 
changing the kernel name you will find it successfully boots the first time 
(since it trys netbsd.vax first)

--Chuck