The standard firmata sketch removes, effectively, the brains from the Arduino. Some other code is actually in control. You want help with the code, you show it.
- Which of the default sketch examples included with Arduino is most appropriate as a base to start editing given what I need to do and the fact I need to use FirmataVB (?)
StandardFirmata is fine.
- How do I edit the sketch to include a variable which lets me control the direction of each motor, rather than setting the OUTPUT to HIGH or LOW (one direction, or stop)
You don't. By loading the StandardFirmata sketch, you've essentially told the Arduino that you are not interested in it's ability to do anything more than read inputs and write outputs.
It is up to the other program that is going to be the smart one to be the smart one.
- Will I need to include other items to achieve this? I've read some stuff about needing to import AFMotor.h or can I just make a few changes to the firmata sketch and FirmataVB?
No, no, and yes.