We have a server side API and a merchant payment API that you can use to integrate custom payments or loyalty into the Vend POS.
Server side API
The Vend API is implemented as JSON over HTTPS using the verbs GET, PUT, POST and DELETE. Or more simply, it lets your web developer (or you if you are a web developer) build cool custom stuff for Vend.
You can create, view and modify most objects in Vend, like customers, products, sales and so on.
The Vend payments API allows you to build your own custom payments integrations in the Vend sell screen.
To do this, create your own custom payment type in the Vend account setup, and under "advanced configuration" you can supply a URL for the hosted payments page. Vend will load this page when this payment type is selected in a sale.
Vend will pass information about the sale using the HTML5 postMessage API (see docs below). You can also update information about the sale using the same API.