Hi Steven
Not sure if when you said “new vehicle payment” if your meaning the AP (Advance Payment) or the one off loyalty / welcome payment (New Vehicle Payment) but as the NVP has not changed I think you mean the AP.
Normally you pay the AP that was posted by Motability on the day you ordered the car. What folk here do if the AP has reduced since you ordered the car is to ask the dealer to put your order through again at the lower AP as its no skin of their noses. If this is what you mean then it will be too late as the price was settled when you collected your new car.
The only other thing I can think you might mean is reading threads here about that when folk are collecting a new car this past month or so there is an option to have the new vehicle payment deducted from the AP price.