However, the action(s) causing the error described is not completely clear i.e.("trying to add to a previous record" Is this something that occurs with update statements, when inserting new rows, or with both types of statements, etc.?). Nor is the exact nature of the DRI and / or other integrity constraints that exist which may or may not be directly involved clear.
The error described i.e.("Referential integrity rules require a related record in table ''tblVendors''"), often simply means exactly what it says.
You may wish to consider posting the table schemas involved, including DRI constraints and the code of any procs involved (sample data to use to reproduce the issue would also assist others in providing more useful feedback).