To add payments to a vendor, click on the vendor card and go to the Payments tab and click on "Add new payment"

Add the payment amounts and the due dates of those invoices. 

Now when you go to "Payments" from the dropdown menu, you'll be able to see what payments you have upcoming to all the different vendors you've booked.

In the example above, we've paid a $500 deposit to the Berkeley Ranch vendor with 3 upcoming payments owed at different dates with a total of $7,500 due.