You can maintain this fields in Excise master data under the t code J1ID, in that Excise indicator for plant if you click you will find the below screen shot. Check in your case also in the same place might have been maintained, if so please take the below logic.
1. VAT number : J_1IMOCOMP-J_1ILSTNO From VBRP-VBELN take WERKS field and pass this information to table J_1IMOCOMP and get the J_1ILSTNO and display this field in to output of the report.
2. CST number :J_1IMOCOMP-J_1ICSTNO (From VBRP-VBELN take WERKS field and pass this information to table J_1IMOCOMP and get the J_1ICSTNO
If the VAT and CST numbers belong to customer then the table and fields would be as below
VAT number of customer :J_1IMOCUST-J_1ILSTNO
CST number of customer : J_1IMOCUST -J_1ICSTNO and the logic need follow accordingly.
thanks,
Srinu.