You need to add vn0c to your "swap on" config line just like you would for
any other device you are going to swap on.

You only need SEQSWAP if you want to treat it as a seqential swap device.
If you want it interleaved in with all the others, you don't need it.

The mount option is probably just broken, try doing a normal mount,
e.g. "mount /dev/vn0c /mnt", after vnconfig'ing (note that you don't have
to "swap" or "mount" in vnconfig, you can just configure.  Even though I
put the mount option in there, I never use it.  I always just use whatever
standard mount utility is needed.  In fact, I used the vn interface to
get the CDROM filesystem working under 4.4 since I didn't have a CD-ROM
drive.

Yes Chris, vnconfig sucks.  I wrote it in an hour, what do you expect!
vnconfig could just configure vn's and nothing more.  I guess I am not
wedded to the "one tool, one job" dogma.  After all, I use emacs!