Too bad gravity effects objects in vacuum, and would pull the anti-matter towards the ground, and prior to hitting the ground it would hit the glass. Not to mention the problem of having to sync of the acceleration of the entire vacuum and the anti-matter inside. Ignoring the fact that the entire concept is physically impossible, there's no problem.
@iago: you're right. A perfect vacuum can never be achieved. Even in a vacuum particles and their opposites combine, explode, materialize, re-explode, etc... over and over. Yay quantum physics.