iPhone controlling iPad remotely: something like Hamachi
There are two questions there. First:

How can I control my iPad from my iPhone

Since you have jailbroken the phone then the answer is to install Veency :

http://cydia.saurik.com/info/veency/ though I'm not sure it supports iOS7 yet.

The second:

How can I get VNC working across the internet?

You will have to configure port forwarding on the network that contains the iPad so that any VNC traffic that hits your home router goes to the phone. VNC usually lives on port 500 and last I checked Veency didn't allow you to change it.