VIN numbers aren’t usually allocated until the vehicle is just about to be built. I haven’t ordered a brand new Vauxhall since the 1990’s, so I don’t know the specifics of how their system works, but the tracking number that you usually need is the one that the manufacturer allocates once the dealer is able to place the order with them.
This is different to any dealer order number or Motability order reference that you might have been given when you placed the order, as these are just admin references and do not mean that the vehicle has actually been ordered. Unless the dealer had available allocations for immediate use, it can sometimes take a few months before the dealer can actually place the order with the manufacturer. Until that happens, there is nothing to track as you are just in a waiting list with no actual order