If you really want to freeze your vodka, you can use either dry ice or liquid nitrogen. Surrounding vodka with dry ice drops the temperature down to -78 °C or -109 °F. If you add chips of dry ice to vodka, the sublimation of carbon dioxide will form bubbles in the liquid, essentially giving you carbonated vodka (which also has a different flavor). Note, while it's okay to add a small amount of dry ice to form bubbles, actually freezing the vodka would produce something too cold to drink (think instant frostbite).