Is the important bit of missing data here the weight of the trailer?
The van can tow: 2000kg
Train weight: 3500kg
Van weight: ???
Trailer Weight: ???
Car weight: 1250kg
Assuming the van, when loaded and towing, is less than 1500kg (check this), that leaves you the full 2 tonne trailer + load weight
So, as long as the full weight of trailer + whats on the trailer is less than 2 tonne AND your van is less than 1.5 tonne, then you should be fine.
Long and short of it:
Everything together must be less than 3500kg
Trailer + trailer load must be less than 2000kg
Unless someone disagrees? Or im misunderstanding