Hey X,
By all means, give vodka a try. Should be very nice, although a little less piquant without the gin. Still, I'm sure it will be refreshing and will take on that wonderful touch of spice from the bitters. Just make sure you have a cigar to go with it! :) I got this recipe out of that wonderful old book of drinks, The Gentleman's Companion (published 1946, I believe). If you can find an old copy of this on eBay or a used bookstore, I guarantee you won't be disappointed. It has much lore, amusing narrative, and some wonderful drink recipes that are truly vintage. This particular gin recipe is wonderful on those hot evenings (I figure the Brits of the empire days ought to know something about this). Let me know how the vodka version is.