Join Existing Data

I think the easiest way to accomplish this would be to create relationships between the tables and use form rules to have the user choose if they are paying a vendor or an employee.

1. Create a pay to field that has a choice of employee or vendor.
2. Create a relationship between Payments and Employees with Payments as the parent
3. Create a relationship between Payments and Vendors with Payments as the parent
4. On the Payments table set a form rule to show:

The other option is to create a payees table and then create a relationship to the employees, vendors, and payments tables and then create an automation to add a record to the payee table each time a vendor or employee is created.