I'm here to answer your query with regard to adding a customer and vendor.
As of now, we're unable to make a customer and, at the same time, a vendor. However, you can put a special character on their name so the system can differentiate which is which. Before doing so, make sure to take note of the customer's name. Then, you can proceed with adding a vendor. Here's how:
From the Expenses tab, select Vendors.
Click New vendor.
Enter the necessary details.
On the Vendor display name field, enter the customer's name (e.g. Customer1-Vendor).