Could you elaborate on
When I create a vendor invoice with an asset posting using transaction FB60, 2 documents gets created which are linked to each other, one for Vendor invoice and an other related to Asset posting. ( Posting key 31 - Vendor and posting key 70 - Asset)
Do you mean "item" when you wrote "document" 
You got no error message in RETURN parameter,
- Was some error raised during commit/update (SM13)
- Which values did you pass to the BAPI (OBJ_TYPE = BKPFF and BUS_ACT = RFBU?)
- Did you use WAIT option in COMMIT, or did you select the document from database just after the commit without waiting for database update?
Regards,
Raymond