VP as Attack Dog?

Every story about Edwards lately seems to be about he’s not an "attack dog" like Cheney, and how the traditional role of the Vice Presidential candidate is as "attack dog". Is this indeed the case? Looking back at past veeps -- Gore, Quayle, GHW Bush, Mondale, Rockefeller, Ford, Agnew, Humphrey, LBJ, Nixon -- I don’t recall this as a prominent theme.

Kerry did give a good attacking speech this week on Bush’s failed war in Iraq. I heard it on NPR, not sure if the establishment media picked it up.