1. Very vague and unknown requirements should be moved out of scope. Use scope control as a way to filter out.
2. Build a quick and dirty throwaway prototype to find answers to the vague / unknown requirements.
3. Identify the vague requirements and ask the product owner the right questions to fill in the blanks.
4. Conduct a requirements grooming session to flesh out the vague requirements.
5. Ask the stake holders to prioritize the backlog and work on the well defined requirements.