I would be careful with tapping the vials to induce freezing. The point of freezing yeast is not to get solid ice crystals, but to lower the temperature. If the sample is at the low temperature of a freezer, then the biological processes are slowed which is why the yeast "survive" longer. Rapid freezing is what causes large ice crystals which damage the cell walls. Supercooling and then tapping the vials to induce crystallization is probably even worse then not using any glycerin at all. When laboratories are preserving living samples, they generally use refrigerators designed to slowly lower the temperature in order to prevent the large crystal growth I mentioned earlier (and what you see when you tap the supercooled vial).
If you are a fan of the effect you can see it on a bigger scale with a bottle of Corona in the freezer, just let it chill for a day or so then take it out and tap it on the counter. Its really cool to watch.