Who is a Customer?
You sell your items/services to customers and receive payments from them. You can create and send sales transactions to them. They can be managed in Zoho Books under Sales > Customers.
Who is a Vendor?
You purchase items/services from vendors and pay them. You can record and send purchase transactions to them. They can be managed in Zoho Books under Purchases > Vendors.
If your customer is also a vendor, then you can link the customer and vendor to create the respective transactions.
Learn more about customers and vendors in Zoho Books from the help document.
and GST filing.