Calling on his conversion rate means you know it as well as I do; he throws away starts. Pretty much why he struggled in one dayers as well.
There have been some unquestionable strokeplayers who just weren't great in the end and plenty more inhabit domestic cricket in England, South Africa and Australia with scarcely more than a couple of matches as a fill in. Mark Waugh was a brilliant player in full flight, but he was dwarfed in overall result by an uncountable number of his international contemporaries. Ultimately, his superb slip fielding was what kept him in the Aussie team as much as his ability to write textbooks with his bat.
Vaughan's best was a remarkable period, but, well, Jason Gillespie has a pretty remarkable PB. You shouldn't judge a player only by their best. In the past 4 years, to the day, Vaughan averages 39 for England (Many would note he averages 38 for Yorkshire), while in the first 16 matches, leading up to that run of devastation, he averaged 31, though anyone would assess that as finding his feet.
In a spell of 26 innings, about one fifth of his international career, he picked up more than 34% of his total runs and 47% of his hundreds, averaging more than 72.
It's one hell of a group of numbers. It actually got me interested in a tangent as to what sort of win-loss records various players' centuries produce, which I'll probably post in its own thread later.
Anyway, what sparked Vaughan's Hun-like rampage and what caused it to end I do not know, maybe he just rolled double sixes, maybe he just was at his peak. Whatever it was, it had a beginning and an end and usually, is not the batsman he is hyped to be. That is why criticism is often encountered, particularly from Australia, considering the number of Australian cricketers whose past glory couldn't keep them in the team, or success in one team that can't keep them in the other team (but also the vast number of 40 average batsmen who never get that big chance).