I have a 147 2.0 Selespeed since April 2001, and I had exactly the same problem you describe. When turning left downhill, the VDC took action at the front wheels. It occured between 60-80 km/h, only when i did not push the gas pedal. The effect of the VDC was like "a big kick in the ass". It had no reason to trigger. I had 17' sports wheels.
I started complaining about this behaviour to my dealer. He changed almost every sensor in the car, because he thought it could be one of them that was failing. But it did not fix the problem. I downgraded my wheels to 15', and the VDC did not trigger so hard. I tried 16', but it was worse than 15'.
After 6 months, my dealer told me there was a problem with all 147 SS, and the problem was the main computer. He also told me that 3 persons complained about this in France, and nearly 50 in switzerland (where I actually drive a lot, and where the VDC was especially sucking, because of many corners)!
Good news, with the new main computer, no more problem!!! Alfa did release in december a new version that was fixing this problem. They seem to change it only if you complain!
I actually have 16' wheels with Michelin Pilot Alpin tyres, and no more "kick in my ass" ;-) And I wonder if I will come back to 17'!
So, ask your dealer for a new main computer.
Now I really love my car!!
I hope your will be repaired soon.
[sorry for the bad english]