You have maintained the vendor number as a partner function. If thats the process its fine but it would not be coiped to the KOMK during pricing because it gets copied from the customer master -> General Data -> Control Data tab. So therefore if you need to assign the vendor number from the parthner function screen to KOMK you need to assign it through user exit as below.
For Sales Order => Include: MV45AFZZ, USEREXIT_PRICING_PREPARE_TKOMK
For Billing => Include RV60AFZZ, USEREXIT_PRICING_PREPARE_TKOMK
- Sachin