You need a single Contacts table. But you will need more than one Occurrence of that Contacts table.
Contacts|Vendor::__pkContactID = Vendors::_fkContactID
Contacts|Customer::__pkContactID = Customers::_fkContactID
Contacts|Vendor and Contacts|Customer would be two table occurrence boxes in your relationship graph with the same data source table. This will allow the same contact record to work as information for a vendor or a customer without any duplication of your data.Tutorial: What are Table Occurrences?