The trick to drilling glass is water-cooling the bit and the piece.
Fill the piece with water, and then fill a dish-pan with water as well.
Hold the piece under water so that the water level is AT the site where you will be drilling. The bit should be constantly wet. The surrounding glass should also be wet, so that it does not heat unevenly, which is what causes cracking. Use smooth strokes and a firm pressure, but don't force it.