The difference between my mt. vernon 7c and my faxx 7c is very noticeable. The mt vernon just has this amazing singing quality to it.
I have no clue about what the specifics are that make a mt vernon so great but mine is definately a keeper.
I going to play the heretic here and say that I believe it mostly boils down to consistency. Regardless of where they were turned out, Bach mouthpieces have historically ranged from the unplayable to the sublime. Faxx mouthpieces are all pretty much the same (and, apart from materials, probably near-identical to the, possibly mediocre, Mt. Vernons they copied in the first place). Moreover, old Bach mouthpieces have been around long enough for many of the real dogs to be winnowed out.
IOW, I suspect your Mt. Vernon was made on a day when the cutters were worn down just the right amount.