So why did it remain a secret here until November 2008?
This has already been proven wrong. Shall I provide the link?
There were plenty of posts discussing parental citizenship regarding a Panamanian-born McCain. There were plenty of posts discussing parental citizenship regarding a "Kenyan-born" Obama. There were plenty of posts discussing dual citizenship in regards to Philip Berg's lawsuit.
As far as the "Vattel definition" that said Obama was ineligible by virtue of his foreign father even if born in the United States, I doubt you can find more than 2 or 3 posts, maybe a half dozen tops (but I doubt that many) prior to November of 2008 --and these posts generated little if any further discussion on the matter. After Donofrio, there'd be dozens of posts each day by birthers who claimed to have known about Vattel the whole time because they learned about it grade school.
The point is that except for a small handful of tossed-aside posts (out of thousands of posts on this forum each day), nobody was discussing Vattel on Free Republic until Donofrio in November of 2008. Then everybody was discussing it.