mine is my computer with a nice big monitor.
work is my work laptop with a vpn into the corporate firewall.
testdt is the computer inside the firewall I want to vnc to.

First, make sure the vnc server is running on testdt.

testdt$ tigervncserver -depth 24 -localhost -geometry 1680x1050 &
(vnc server started on port 5901)

Next, forward ssh from my computer to work's computer to the vnc server host and then tunnel the vnc port from above over that tunnel.

mine$ ssh -NYL 2222:testdt:22 work &
mine$ ssh -NYC -p 2222 -L 5901:localhost:5901 localhost &
mine$ open -a /System/Library/CoreServices/Applications/Screen\ Sharing.app

Once Screen Sharing starts up connect to localhost:5901.