Why am I getting condensation on vapour barrier?
I don't see that there's anything wrong. Condensation happens... on any surface that's colder than the dew point of the adjacent air. If anything, it's showing that it was worth doing. Despite only being a few mils thick and easily heated, it's much colder than the inside air due to heat loss through the windows. Without it, the windows would be sucking heat out of your house instead of just the interstitial space. The only thing you can do to reduce condensation is reduce the humidity in your house and/or turn the heat up.