There's an interesting interview (the "Mugglenet/Leaky Cauldron" interview) in which the interviewers suggest to Rowling that perhaps Dumbledore tries too hard to see the good in others, specifically with reference to Snape. Rowling "agrees" in her usual noncommittal manner. We shall see... in a bit less than a month now. (My copy will be a bit late, as I ordered the Canadian version to get the recycled paper, and so they have no obligation to deliver it to me on the release date. The fan-generated ebook version will quite likely become available before my copy arrives.)
I've always wondered when Snape's birthday is. He was, in fact "approaching" as Trelawney was spewing out this particular prophecy. That's probably not the direction Rowling is headed, based on her interview comments, but it would be amusing. Rowling has said, however, that the reason the prophecy is true is that Voldemort has made it true by choosing to attack Harry, not that there was some inherent fate involved.
I don't think any of Harry's previous encounters with Voldemort qualify as "vanquishing," though. I agree that "vanquish" doesn't necessarily mean "kill," but it seems to me that if the villain comes back in the very next book of the series every time, the term "vanquish" isn't quite appropriate.
The other big mystery character is Aunt Petunia. We've been promised more info on her for years, and there have been hints, but no real explanation as to why she has harbored Harry all this time, considering how much she claims to have loathed his parents, and considering that she seems well aware that having him around puts her own family at risk.