'APPrOaCH category is for straight in and is based on Vref (vat 1.3vso or 1.23 vsr whichever is higher)
CIRCLING approach category has nothing to do with VREF (your aircraft aproach category), it is solely based on the SPEED you are going to fly it.'
I would like to see a reference for this from the country that you're flying in. The only references I have seen define catagory on Vref (based on stall speed at max landing weight).
The wording differs from country to country but most allow operators to choose to fly at a higher catagory to allow the aircraft to be operated at a higher speeds during the approach. Everywhere I have flown also requires you to use a higher catagory if you have a malfunction that drives your Vref up. For example if you are Cat B and have a flap failure then your Vref may now be in Cat C.