Teeth in the vagina wouldn't be that dangerous, unless said women also had maxilar like bones in their vaginas too that would force said teeth into the guy's dick.
Heh, now that sounds really scary to me!
And a little familiar too, check out this solid horror-comedy if you haven't before: http://www.imdb.com/title/tt0780622/
Funny how vaginas with teeth are scary, but getting a blowjob, which puts your dick between a pair of bone knives controlled by powerful muscles mad specifically to cut things, is still considered hot.