I attended two, once as an applicant and once as an appointee who accepted the appointment. I thought the second CVW was more valuable than the first. My mindset was different between them. I felt like a guest just visiting for the first one (which I was), but the second had a different tone. I had a general idea of what USNA was about and how it operated from the first CVW, so I was made my second visit a bit more deliberate. I was more focused on asking about plebe life and envisioning myself as mid more. I felt that the plebes I was with were also more candid with their answers to my questions since I was already committed.
TLDR: I think a second third to Annapolis is worth it. You can ask more poignant questions and really take the time to see yourself as a mid. You can ask yourself if you truly want to do it.