Is a product vision statement worth user testing?

Product visions should always be customer-focused. I'm not sure that you need to actually show your vision statement to users to get their feedback, but your vision should be informed by the user problems and product goals that your team hopes to solve over the next few years. In order to do that, you need to know your users intimately, and that often requires customer research and reviewing product data. 

Yes. At some point, you want to have a set of key workflows and key personas nailed as a part of the vision canvas. And using low code or UX mocks etc. - you want to test the validity of the hypothesis to the next granular level. Always be testing, always be validating, always be pivoting and making small tweaks to your plan based on the feedback and data you receive.