You might take 20% off but the vat cost to the dealer is nowhere near that. They pay vat on the sale but recoup it on the cost price the VAT is only on their margin. The 20% plus 5% a year figure might be close but its due to the dealers margin (fair enough they have to make a living) not VAT. It their margin on selling is say 15% the vat will be about 3% of the vans valueI must admit if i look at a used private van for sale my way to value it would be to find the retail price, take off the 20% vat and then 5% per year. So 25% loss in year one... The longer you can keep a van the better, however new prices shoot up so just costs more for the next one.
A December purchase of a stock van with 20% discount means the shock will be taken out on my current van BUT have lost loads on past vans...