How about a secure async chat where the vendor simply stores a list of message IDs, and then the client requests if anyone has a copy of any message you haven't received yet from the other users in chat when you log on
Such vendor would have a hard time finding a business model since plenty of chat-services are already existing on the market and all of them have access to the data of their users in one way or another. Thus I don't know what other type of leverage they would be able to pull off to sustain their business.