vodka's a really bad idea, I tried another strong liquor once as a prank, and my friend is now covered in scars from when the liquor caught fire and blew up in his face.
Vodka's at most 80 proof, too weak to burn. Hell, using vodka (or other liquor) instead of water is one way of smoking a hookah (which, for anyone who's unfamiliar with the wonderful devices, is basically an inverted bong: you have a ceramic bowl on top in which the shisha (basically, shredded tobacco leaves covered in flavored syrup) is placed, covered in a filter on which is placed hot coals, with a hole in the bottom connected to a pipe that runs down the stem and into a bowl of water; the tubes you draw smoke from are in the air above the water, so when you inhale it reduces the pressure above the water, pulling air (and smoke) through the central pipe so it bubbles through the water (or vodka, milk, juice, whatever) and gets drawn through the hose), though of course it's dumb as shit to try to smoke pot through one (not least of which because that leaves traces throughout it, making that nice hookah highly illegal to possess). Of course, the alcohol is much further from the flame (which is itself a cool-burning coal, rather than burning butane as which a pipe or bong) in a hookah, usually more than a foot, often two or more.
Then again, I don't really see what adding vodka to it would do, aside from give him a light buzz from the alcohol... Or what tonic water would do, for that matter.