Aircraft are certified by the manufacturer for the Approach CAT.
Vref is based on several factors, with a baseline. Different weight and flap settings influence the actual value, and many operators will use CAT D for a CAT C aircraft when calculating an approach...
As an example, the Vref CAT C for a 737 is based on 40T LW with flaps 30...
The pilot must calc actual LW and flaps to determine to use the CAT C or CAT D approach...