Has the dealer informed you of a price increase by way of a formal communication advising the price increase to your order (order variation) as they became aware of the variation at any time during your long wait ?
They can’t simply say here is your vehicle now pay a different amount to the agreed order value, unless they have formally advised you of the price variation(s) at the time the price was varied By VW.
I.e when VW changed the price & advised the dealer.
If the dealer has not advised you of a variation to your order (at the time the variation happened) they cannot claim to have honoured their side of the contract and similarly they cannot claim a variable price increase from the agreed order Value.
simply put the dealer has Zero risk and you (the customer) burdens the whole risk, which is an unfair contract, unless the dealer has complied with the variation notification To yourself when it happened.