For many cases the exit RV60AFZC – USEREXIT_FILL_VBRK_VBRP is not working for VAT number update. This is due to the config of Sales Org. Table TVKO have the Sales org config which stops the update of VAT number. Based on entry it picks the VAT ignoring the code in the exit.
Use below UserExit depending on the config from where the VAT is picked