From what I've heard anecdotally, if you knock it out of the park at NASS (do well on CFA, are a real team player, etc.), it can be a TINY help. About 10 years ago, I had a candidate who had done the above and USNA knew about it (they tom me the candidate was the top performer at NASS, whatever that means). Candidate was offered an appointment but turned it down.
If you are a total "negit" (pretty much as described above) and the write-up from your PS detailer (who is just a brand new 3/C BTW) is generally in line with what Admissions sees from teacher recs, BGO write up, CFA, etc., it could hurt a bit. But that candidate is probably doomed based on other input.
Overall, it IS a recruiting tool and for the overwhelming majority of candidates, the primary outcome will be helping the candidate decide whether to continue his/her application. In that regard, I just had a candidate who attended NASS email me the "they" (pardon the grammar) are not continuing with their USNA application.
I don't think an Official Candidate would get a CVW invite unless admissions thought you measured up in some way ...
This used to be the case (as in 40 years go). Not today. If you're an "official candidate," that is typically enough to get a CVW. There are usually more slots available thank taken. I've had quite a few candidates in recent years who had CVWs who received turndowns. Great candidates, but the numbers worked against them.
I realize many candidates who are appointed may have done CVWs, but please don't read too much into an invitation. With so many people not being able to attend NASS, CVWs are offered as a means for those folks to see USNA.