Odoo recognizes a contact as customer or supplier if his supplier_rank or customer_rank is > 0. By default, when creating a contact, the rank is = 0.

There are 3 standard ways to update the customer_rank or supplier_rank increases in V.13 and V.14:

1. Invoice/bill the contact. Each time the contact is invoiced, the customer_rank increases. Same goes for the the supplier_rank when billed.  

2. Create/import the contact via the sale/purchase app. The rank will be automatically set to 1. 

3. With an import in the contact app, force the status using the field supplier_rank = 1 if supplier and customer_rank = 1 if customer. 

You can always customize this by creating a new checkbox "vendor" and "customer" and adding an automated action that would set the rank = 1 if checked, if so, don't forget to create the reverse action that sets it to 0 if you uncheck it. But most of the time it's just redundant since the standard works perfectly.