When I get asked about Vision and Purpose in a business, I believe vision is the ability to create a picture, or visualise, what the business looks like in the future. Purpose is a cousin of and interacts with the vision. I believe it answers the why, (a reason) for the business to exist.
If there is not a clear statement of purpose, the vision may have difficulty converting into actuality. There is less motivation for people to move the business towards the vision.