Andy from The Jerx points out that the traditional conjurer's technique of tapping once, twice, thrice-and-VANISH is a fundamentally unsound way to do a vanish, because you're showing the audience what the tap looks like when you're doing doing the move, and no matter how much you practice that move, you'll do something a little different that third time.
Instead, he suggests tapping somewhere else once or twice, then striking your coin (or whatever) on the third tap.
I do think it's a good idea to establish the idea that the coin is going to vanish when you tap it, but instead of doing two practice taps on the coin and then a different looking tap to vanish it, I tap somewhere else. "The coin will vanish when I tap it," I might say, tapping the air in front of me. Then I do the striking vanish. Because I haven't established a pattern/rhythm of tapping the coin, there's no pattern or rhythm to be broken.
To Make the Vanish More Striking