You might want to check out this howto
I know it says smoothwall but it should work on M0n0 too. I've successfully
used this method with several other firewalls, haven't tried it with m0n0 yet.
Quoting Cheyenne Deal <deal dot cheyenne at gmail dot com>:
> How do I configure m0n0wall for incoming vnc connections to a computer(s)
> behind it? I knew using a regular router but now here I do not know how?