Hello
Function module MD_MRP_LIST_API is not released (you can check the status on SE37), which means that it was originally developed to be used on SAP internal programs and transactions.
Therefore, you should go for the BAPI BAPI_MATERIAL_MRP_LIST, which was developed specifically to be used on custom programs.
SAP takes special care with BAPIs, in order to keep the interface compatibility when making changes or improvements, while the interface of a function module that is not released can be changed, which could cause problems on a custom program.
Now you have mentioned that you will not use the material number as an input parameter. Therefore, before calling the BAPI you should select all the materials of this specific MRP controller and use that as an input parameter to the BAPI. Later, you can filter the BAPI results by vendor.
BR
Caetano