Just having a look at his profile (Michael Vaughan | England Cricket | Cricket Players and Officials | Cricinfo.com), and then his series by series record (Batting records | Test matches | Cricinfo Statsguru | Cricinfo.com), where he had a couple of years where he was without one of the best batsmen in the world, and then his form started to drop off, with the exception of the West Indies, India (2007), and New Zealand (2008), his averages left a lot to be desired.
His Test average finished with an average of 41, but his FC average is just under 37, so apart from his purple patch early last decade, he wasn't as consistent, so is that a case of his Test career moving more in line with his FC record, and therefore an overachiever at Test level. Certainly a good Test player, not a great one.
He should never have played 86 ODIs, I know he was captain, but I guess that evidence that England didn't have anybody better, and, they didn't want to have two captains, one for the Test team and another for ODIs.
Injuries have also hampered his career, and he missed the 06/07 Ashes as a result, perhaps he could of made a difference, but Australia still would have won IMO.