Vendor-This is an outside vendor that you use to print jobs that your customers have ordered from you. The order comes to you from either a retail site, or a customer storefront that you have set up in the system. You see that order in your admin area, choose "edit item" and choose the vendor to send the order to. This is what you describe below.
Broker-This is when you have a broker that orders from you in behalf of his customer. You can either set up a customer storefront for your broker to access and order directly from you, or you can set up a customer storefront for HIS customer. You would brand that customer store to use the brokers logo instead of yours, along with his customers logo. This storefront would have the products you print for this customer, they would login and order products from you. Your broker would be notified of the order that you received.
Please note: You are the one that has the subscription, the only one that receives orders and payments and the only one that accesses the administrative side of the system. You are able to build an unlimited number of customer storefronts... we encourage you to set up as many of your repeat customers in those storefronts so they can order directly from you in their own branded site. The broker interface is nice because it allows you to extend that same benefit to your brokers customers.