It will intervene if it detects a certain amount of slip from the wheels. That exact figure I don't know, I'd guess around 3-5% from experience.
In your example, you have to remember that weight transfer is key. In the CTR, if you go in to fast then all you do is understeer, as the wheels cannot turn and transfer all the power at the same time. To fix this, you simply lift off a fraction. In the Zed, if you enter too fast then the back end will overtake the front as that's where the power is coming from, assuming a steady or increasing throttle. My guess is that you braked/lifted as you entered the corner, and then got back on the power too early. This caused the rear end to light up, which the VDC quickly decided (rightly) was a bad idea and so cut power. Basically, and without sounding too patronising, you're going to have to learn how to drive!
RWD isn't easy, especially if you've come straight from FWD. It does take time, and after a while yes, you will be able to go quicker with VDC off as you actually want the car to be in a neutral four wheel slide as you enter a corner, but if you're still finding the VDC kicking in unexpectedly then I'd be leaving it on for now.
That said, in your specific case check the basics. What tyres & sizes do you have on? Pressures? Tread remaining? What geo are you running? Got the spare wheel still in? It might well be something fixable rather than your own skill.