Martin , you do not say what year yr Vectra is or what model , however the 85% rule is a used as a good guide to getting the balance right between being able to tow safely and not having the effect of the " Tail wagging the dog " if you have too much weight in the form of a van on the back. It is always recommended that those new to this enjoyable pastime do not exceed this limit when " cutting their teeth " in towing.
If you load the Challenger up to its max weight then the tow car should have a kerb weight of around 1650kgs to get to 85%. do not be fooled by the max tow weight that the vehicle will tow based on what the manufactuer says , you should never tow anything that exceeds the kerb weight of your tow car.
Based on the lowest kerbweight listed for a new vectra at 1431 kgs you should be aiming for a van with a mtplm( its max weight with all your sleeping bags clothes, gas bottle etc ) of 1216kgs this would get you to 85%
I tow with a 2.0 Vectra at 89% if the van is fully maxed up to 1250.
The kerbweight of a car is defined as that of the car , driver and full tank of fuel.
Extra persons / luggage / etc do not count but do help to put more weight at the front of the outfit when loaded , but do not use this additional weight to help you get to 85%.
Hope this helps , as we have all been through this minefield when first starting out.
rgds
Pip