A solution would be to create a Custom Node Property that contains the Vendor name.
Instead of having an empty field for the ICMP devices you can use the below SQL command to update all of them at once:
SET CustomPropertyFieldName = 'NONSNMP'
WHERE ObjectSubType = 'ICMP'
To update the SNMP nodes, you can use Database Manager and copy/paste the initial field information to the Custom Property.
You will then be able to sort your nodes using the custom property instead of the default vendor field.
Note that if you try to automatically edit the Vendor column, changes will be overwritten after sometimes hence the Custom Node Property requirement.