Sounds like you are on the right track and shouldn't be far from connecting successfully. I am not in front of my PC which is running VNC so I can't give you exact instructions right now, but essentially the steps are:

You need to install a VNC server on your PC, like RealVNC
You need to ensure the server is running
You need to enter an access password
You need to set up port forwarding so that your router forwards the correct port to the PC running VNC server

Then you can install VNC on your N800.

When you start VNC on the N800 you need to tell it the connection details, ie, what address you want to connect to. If you are on your local network, give the local IP address, like 192.168.1.3. You might need to add the port number as well I can;t remember. If so, it would be 192.168.1.3:5900 or whatever.

If you are accessing your machine over the Internet, give the IP address of your router. If you have a static IP address, then that's easy. (You can also give the full host name, like fishead.btinternet.com)

If you have a dynamic IP address, then that means that your IP address changes each time your router connects to your ISP. If you have a dynamic IP address then you need to go to somewhere like dyndns.org, which offers a free service and allocates you with a host name like fishead.dyndns.org which automatically updates itself with whatever IP address you are assigned when you connect. Effectively this means you can always enter fishead.dyndns.org into VNC Viewer and it will find your router, and thus your PC.

I guess I can't help further until you tell me at what point you are encountering problems. But I am sure we can get you working - it's not that complicated although it seems so when you start!

As for VPN - that's an encrypted link between two machines. You can run a vnc session over a VPN. But you can also run a VNC session over the Internet without using a VPN.

If you want to find out more about setting up OpenVPN check out my blog (there's a link at the bottom of this posting)

Once you get VNC working to take control of your PC from your N800, you could set up OpenVPN, making a secure link between your N800 and your PC, and then connect the VNC client to the VNC server over this secure link. In theory this should be a secure way to access your PC from the N800.