If you have a jumpstart server on the network, try:
ok: boot net -s
mount -F vxfs /dev/dsk/c0t0d0s1 /mnt
You'll need to manually mangle /mnt/etc/shadow, as any passwd commands would be related to the "current" os instance, rather than the mounted instance.
Disclaimer: I've not tried to do this with a vxfs volume
Edit: You'll have to follow normal configuration rules for adding a host to the jumpstart environment - updating bootparams
and of course, this only works for SUN gear...