Being the volcano groupie that I am, I've spent quite a bit of time studying the 79 AD eruption of Vesuvius. Pliny the Younger gives an eyewitness account of the eruption, which includes a description of the death of his uncle, Pliny the Elder.
Until today, I never have heard of it being pronounced "ply knee." I've always heard it spoken as rhyming with Vinnie.
I mean, come on guys... Can you really hear some Italian guy saying, "Yo, Ply Knee!"