If your van has differing VINs then it potentially has been tampered with previously and could be illegal to use on the road. We would suggest gathering all the documentation you have available from when you bought the vehicle, including the sales agreement or invoice and MoT certificates before contacting the original seller.

If it was a private sale then unfortunately your options are limited, but a dealer should acknowledge the issue as it could be potentially damaging for them. Ask them for a solution - they may offer an exchange vehicle or to purchase it from you. If they refuse to cooperate then you should contact Trading Standards.