I'd agree with this. There's not enough evidence to support the assertion that Vaughan is one of the best players in the league. I don't think too many people would seriously suggest that.
What I will say, though, is any time Vaughan has been out for a while, I've started to think "Is he really that good? Is it a case of a player getting better in the minds of the support the longer he's out?" and then he's been even better than I expected.
That has largely been at a lower level, although as you say his hattrick in the Cup was pretty compelling evidence.
He's got 13 goals in 73 Championship games, which isn't particularly impressive, although that does include all of his breakthrough game time. In League One, as a more mature player, he's got 21 goals in 46 games. For a player who isn't even an out-and-out striker, that's pretty good going.
I think with most forwards at this stage of the season, you look at them and say "Can he get ten goals this season?". With Vaughan, for me, it's about games. Can he make 25 starts? If he can, I'd be very confident about him scoring or creating (depending on his role) double figures.