Sorry, I obviously didn't make myself clear. If, as you say, they offer different towing weights and they show different train weights on the VIN that is as it should be. As long as the combined weight of the van and trailer does not exceed the train weight, individual axle weights or the limit of the various parts of the towingf hitch then the combination is legal. So,
In effect you can carry less in the tow vehicle so that you can tow a heavier trailer. This is a lift from the Govt web site
7.3 GCW - Gross Combination Weight
This is specified by the manufacturer of the towing vehicle and it means the maximum permitted weight of the combination (ie total of the towing vehicle and the trailer, including the loads on both). Also known as Gross Train Weight (GTW). To help find the maximum weight of trailer that can be towed, you could subtract the GVW of the towing vehicle from the GCW. (in this example a 2000kg trailer would be permitted). However if the towing vehicle is not fully laden, any spare weight capacity can be used by the trailer, so long as the GCW and trailer GVW are not exceeded.
Towing trailers, caravans and horseboxes with a car, including the weight and size limits, and what licence you need to have
www.gov.uk