3) check billing document field (BSEG-VBELN). When you find the billing document number (VBELN), you'll find the corresponding VKORG, etc. in table VBRK. RFPOS is a structure that might be used for both customers and vendors, so I think that KONTO = KUNNR when you are looking ...
TASSIGN_MM_T001W table in SAP MM (Inventory Management in MM) module. This tables is used for storing data of Aux. Table for Status Info SD Maintenance View: Key VKORG. Here is it details, table fields, field types & length, technical data etc.