1999/2000-2000/01 - promising but still plenty to be done.
2001 summer and Indian leg of 2001/02 winter - looking like building on promise but interrupted by misfortune.
2001/02 NZ tour to 2003/04 winter - opening the batting and wasted there (even though he played some good innings in the summer of 2002 and winter of 2002/03 and one in the summer of 2003).
2004 summer - pretty good
2004/05 winter, 2005 summer and the 1 Test in the middle-order in 2005/06 winter - pretty poor
2007 summer - good (in fact as good as I ever saw him)
2007/08 winter - wasted opening the batting again
2008 summer - poor
So in short Vaughan, like Ian Bell so far, never really got a proper shot at fulfilling his best role for very long, but because people expected him to be more than he probably could be (due to the attractiveness of his strokeplay). But on the few occasions he got the chance in his best role he didn't really convince quite enough to make it look obvious that he was wasted elsewhere.