What does "shell account" mean ?
- Can you ssh to it ?
- Does it allow port forwarding ?

If so, you could forward your VNC-server's VNC-port to your "internet
computer" INETGW. There you could connect from anywhere and attach to
the forwarded port.

On your VNC-server (asuming you are using port 5800)

ssh -R 5800:127.0.0.0.1:5800 INETGW -Nf
-> simply open port 5800 on INTEGW and forward it to your localhost
port 5800.
-> -Nf simply doesn't create a remote shell, only the port will be
forwarded. You will not have a shell on INETGW.

Now from any computer:

ssh -L 5800:127.0.0.1:5800 INETGW -Nf

-> this forwards the remote port on INETGW (which in fact is the
forwarded port from your VNC-server) to your "any computer".

Now on your "any computer":

vnc localhost
-> this would connect you to your VNC-server via the INETGW.

Best regards,
Armin