How to mount mfsroot?

[Yuri]

I am trying to set up NFS install, and need to modify mfsroot to not 
load sysinstall.
But vnconfig is disabled in 8.0.
http://www.freebsd.org/doc/en_US.ISO8859-1/articles/pxe/article.html 
suggests using "vnconfig vn0 mfsroot"
But vnconfig is disabled in 8.0.

So I ran:
mdconfig -a -t vnode -f mfsroot
mount /dev/md0 /mnt/
And got an error:
mount: /dev/md0 : Invalid argument

What's wrong?
Yuri