I just have a short discussion with the development team on this topic. They confirmed me that it’s really a bit difficult right now in WP ERP to add a new option (to add contacts) under “Vendors”.
But I have another suggestion for you. You can create several groups under “CRM -> Contacts Groups” and add multiple contacts there. For easy understanding, you can give the group name as same as the vendor name.
This may help you. Thanks 🙂