Hello, I am Keith, one of the experts on Just Answer, and pleased to be able to help you with your question.
If the vendor is registered for VAT and the van is on their books than they must charge VAT on the purchase price of the vehicle. Of course, they could agree to a figure and then put it through their accounts and net out the VAT, but whichever way it is sold then VAT will rear its ugly head somewhere, either added to the selling price or absorbed by the vendor. Of course, if you are registered for VAT the tax charged is an input and can be reclaimed in the usual way on your quarterly return.
I do hope that my reply has helped clear the air,