What you need:
1. Apple iPad VGA adapter
2. USB cable with one end 4 pin USB A jack connector
3. Soldering iron with small tip
3.1. Hawing flux will help a bit.
3.2. Some tin
Step 1: Opening 30pin connector
Step 2: Remove shielding
Step 3: Customize shielding plates
Step 4: Solder back one plate
Step 5: Solder your USB cable
2. Make hole in plastic case for wire and put wire throught it, don't miss it!
3.Solder USB wire as in picture (F-Free)
4. Solder USB shielding somwhere to metal casing (if cable has one)
USB pinouts: http://pinouts.ru/Slots/USB_pinout.shtml
30pin connector pinouts: http://pinouts.ru/PortableDevices/ipod_pinout.shtml