If you want glass in a vocal booth, its true a lot of people use a double-pane window, tempered or not, however usually the glass panes are the same thickness, and in true parallel. Therefore, sounds inside the vocal booth will transmit vibrations to the first glass pane, through the air "chamber" between the two pieces of glass, to the second glass, then come through into the room facing the vocal booth (console room usually).
The two pieces of glass really should be different thicknesses so they do not share the same resonant frequency, and one should be perpendicular to the floor, and the other at a small angle so the air chamber inside does not have parallel "walls" of glass. This simple construction technique seriously cuts down in acoustical transmission from the booth to the control room, if that is what you are looking to do.
Making a window yourself is really not that difficult. I chose not to make from scratch, and instead I bought a double pane window as Ms. Hilarious suggested, then built another wooden frame for the angled piece of safety glass, and have it ready to screw into the wooden frame of the purchased window. Cost me about $50 on top of the cost of the purchased window.
