The Vendor field is edited by the polling engine.
To avoid conflicts, you should create a Custom Property, update its fields with a SQL Query based on the Vendor column. You can use as well a CSV export to populate the new custom property.
Once done this edit the remaining nodes that do not contain a Vendor value if you did not use the CSV method.
Configure your resources to be grouped using the Custom Property instead of the default SNMP Vendor Value.