You might want to check out this howto
http://martybugs.net/smoothwall/puttyvnc.cgi

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.

VinceV

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?
>