Having vision for a product means correctly identifying customer's problem and proposing a solution for it. Dropbox vision is: "Make user files accessible anywhere, by storing it in the cloud synching them across devices". Google's vision is: "Make all information in the world easy to find and access, by having search engine know you better than you know yourself" :)
Customers either buy into your vision, or ignore it, and the latter means that the problem that you stated does not exist or is not important enough to your users.
For you, having clear vision helps you create MVP, because you can easily identify which features are crucial, and which are nice to have.