Every batsman has the fortune of catches being dropped-hell the aussies won a world cup because of a dropped catch-I wouldnt consider it at all.
I see the dropped catches as an irrelevant argument - every player in history has benefited from dropped catches, its part and parcel of the game , you cant cherrypick the stats to say "oh if he had been caught....".
Good players make the most of their chances and you don't get to average 51 after 57 tests unless you are a reasonably good player. Sorry, luck by itself or in a major way does not get you numbers like those after that number of matches as well.
It's very stupid to say dropped catches don't matter at all, IMO, nothing short of. Of course some batsmen have more good fortune than others, and of course you cannot possibly consider scorebook records as equally valid when one batsman has notably more fortune than another.
Sehwag has of course been far more fortunate with dropped catches (mostly against Pakistan but also against Australia and others) than almost all batsmen are, and I don't think he's all that good a Test match opener. I do think he could be quite a good middle-order batsman, though. And funnily enough the same thing is true of Vaughan - as an opener, he's received a fair amount of good fortune which makes a pretty poor record look rather good. But as a middle-order batsman, he's good.
Sadly, though, as has been pointed-out, Vaughan has been nowhere near as good as he could've been. Started his Test career aged 25 (even that was perhaps a bit premature) and didn't really kick-on until he was nearly 27. Then wasted most of his best years (and another 6 months later) opening, and had another 18 months taken by injury. Vaughan, in my view, is someone who could've averaged 50+ had he batted at three and four all his Test career. Sadly, though, he didn't, and he'll have to settle for being merely good - barring a remarkable twilight to his career.
So really, I don't think you can offer much of a comparison. Sehwag is someone who has been extremely fortunate (not just with dropped catches but with rarely playing on a seaming pitch) to make it good as a Test opener, Vaughan is someone who looked like the same was going to happen but the fortune dried-up.
As middle-order batsmen, Sehwag has barely had a chance to show his skills, Vaughan has been just a little disappointing. It's not really a comparison I'm willing to be making.