I concur with markflip. Remember that the 'VAT' reduction is from the List Price of the car, not from the price you have already agreed to pay. I suspect that you will have negotiated some discount already from the list price and that this won't be a million miles away from the new offer.
Manufacturers change their offers generally on a quarterly basis and using Headlines like 'No VAT' helps to keep their marketing offers 'Fresh' in the eyes of potential customers. Its a very clever way of making people think that this is a better offer than has ever been on the table but, if you read between the lines, the amount of money you need to get out of your pocket is usually very similar.
Again, don't let this ruin your enjoyment of your New Car - especially if there is only a small amount of difference between the offers.
I bought a TV last year for somewhere in the region of £1500 and while I was buying a Washing Machine in the same place 2 months later, my TV was less than a grand. Ho hum. I still love my telly though...
JJ