Vag com is diagnostics side, which has got extra's such as changing the beeping when locking. Idle speed adjustment and throttle alighnment, which requires to login to the engine. Don't know its full potential as I'm always learning things.
Vag tach is re-known for the glowing needle, MTE and changing clock milage. But people have downloaded the hex files and done alot more such as adjusted the temperture needle not to stop at 90 (as it stays at 90 but in reality its constantly shifting a few degrees plus and minus), so they adjust the temp needle to show real time temperture which I can see as useful possibly for racing... Again there's alot to do and alot I still do not know about on vag tacho.