There are a few clarifications needed here, before I can help you out in the right direction, as per your query.
1- Are you working in an Agile/Scrum model?
2- Why are the 'Requirements' unclear?
3- Why do the user stories change so often?
4- Why are the user stories so vast?
I think you may find that your testing is hampered, and you are being set up to fail by bad process.
In general, User stories should be small- less than 2 weeks worth of work to deploy to production. Requirements (acceptance criteria) should be clear and unchanging for the most part.
If these variables are controlled, you won't need acceptance test planning, just testing of the story to the satisfaction of the Product Owner so they may 'Accept' the User Story and deploy it to production.