A vendor is a distributor of ApolloTV. Typically a vendor will have their own server software and range of companion client applications. Vendors are not officially endorsed or supported by ApolloTV.
Fundamentally, all you need to become a vendor is your own range of the ApolloTV applications. This means you can just start your own server and/or write your own configuration and get started.
If you want to learn how to write your own configuration for a client application you'll need to visit the Wiki for that particular application.