Are you sitting down, Rogues? Ghostcrawler finally addressed the Vanish bug over on the forums, but not in the way you'd want him to: he says that while it probably is a bug, they can't seem to find it. According to Mr. Street, every time they sit down and try to recreate the bug issues with Vanish (aggro isn't lost when it's supposed to be), they come up short, like a mechanic who never hears that rattling in your car.
I'll echo what many players in the thread say: it is there. I've had it bite me many times -- given that I don't Arena much, it's usually when I pull aggro from multiple mobs while soloing, and try to Vanish away: one of them will strangely keep attacking me. But in terms of reproducing it, GC's got me -- I don't know when it happens, or what circumstances make it happen, I just know that it happens. And when it does, it's annoying.
He also says that like Onyxia Deep Breath-ing more, the Vanish bug has moved into a kind of legendary perpetual problem, so much so that they can't tell the real complaints from the jokes. So maybe for your favorite bug in the game (ahem, someday I'll get a Cower bug fix), it's not so much a matter of Blizzard just fixing it, as Blizzard finding out why it's happening in the first place.