There is no option. If the buyers get acknowledgement, that should be reflected in some place. Suppose that you have your measurement system in an excel, then the problem is the same: if the buyers don't adjust the date, you will not be able to trust in the vendor scores.
You can use a user exit to adjust the statistical delivery date automatically when the delivery date is updated, but that would not be a good decision, because the statistical delivery date should only be adjusted, if the reason why the delivery date is modified is because your planner decided to do it and it was agreed with the vendor, and not because there is a delayed and the planner is just adjusting the document to communicate the vendor delay.
Kind Regards,
Mariano