It depends on your rules and definition of "processed successfully"
I assume that you are referring to the Message Tracking application.
Message Tracking's definition of "processed successfully" is that the MIF processed without generating any errors.
you may have a processing rule that means that the request is skipped when a particular value is provided - in that case the vendor is not created AND no error is generated.
you may want to set the integration logger to DEBUG and then reprocess it.
the entries in the Systemout.log file should give you an idea about what is happening
a me