VNC Server (veency on the iphone) is a service that runs in the background and accepts incoming request from VNC Viewer. The default port it uses is 5900 unless you change it. To get the request to the phone you either have to be on he same subnet (same wifi network) or have a router that will point traffic from another network (normally the web) to the IP Address of the iphone.
Example your phone is connected to your home network through wifi and you are at work and want to remote into it. You would set up a port forward in your router to forward incoming port 5900 traffic to the IP Address of your iPhone. The in VNC Viewer you would enter your router's IP Address and it would forward you to the iPhone.
However, I have a feeling you just want to be able to remote into your phone regardless of the network settings. This would require a broadcasting service running on the phone and I do not know of one at this time.
For information on the "type" of broadcasting service I am referring to you could read up on how LogMeIn Rescue works. However, they currently do not support the iPone. LogMeIn Ignition does however support accessing any computer from you iphone. I personally use VNC Mocha Lite to access my PC from my phone.