The main differance between Brake shudder and the ABS kicking in is pretty huge.
If your doing 80 and stand on the brakes, the pedal will pulse quite hard, moving about a cm in and out and the car would come to a stop.
Brake shudder is more like a soft pulsing of the pedal while using the brakes. It could be a warped disc moving the pads back and forth in the calipers forcing fluid back to the pedal, hence the pedal pulsing.
One of the reasons why cars, well, actually drivers with ABS have accidents is becasue as soon as they feel the pedal starting to pulse, they back off the brakes - when they should stay standing on it.
Just FYI, I tested my old car, an '01 Vectra, in the rain doing 80 on an unused road, the car stopped within around 10 metres.
Have yet to test the VE....
I hope this helps.