I'd taxonomize levels of Vintage mastery into three levels:
Competent: You understand how your deck works, its strategic objectives, and that of your most common matchups. You make few tactical errors and can recognize and capitalize on those of your opponent.
Skilled: Beyond mastery of your deck's basic tactics, you can draw inferences about your opponent's likely hand, deck, and sideboard from the flow of play, and play around the most likely threats and to your outs. Your experience with the metagame and specific matchups is deep enough that you comfortable with the deepest aspects of Vintage play: sideboard construction, sideboarding strategy, mulligan decisions, and (as per wfain) proper sequencing.
Master: You can "read" individual opponents, and adjust to their style and psychology of play.
It's important to stress that Vintage players are, for the most part, excellent players of the game, despite the complexity of the format. The average Vintage player would probably be considered "very strong" by the standards of the broader Magic community: patzers don't invest the time and money to play Vintage.
It's hard for me to pinpoint specific plays that identify a Vintage master, but I know I'm facing an excellent player when they play as if they could see my hand: countering the right threats, naming the right Therapy targets, etc. It's easier to spot negative evidence of mastery: tapping the wrong lands, mistiming spells, poor threat assessment, dubious sideboard decisions, etc.