From what I've been reading it may be rather dangerous to drink on ketosis, though views on this are contradictory.
It's the "not all calories are equal" part of Dukan. Just as Glucose and Fructose are much more easily "burned" by the body, than Lactose causing spikes of insulin etc - Alcohol is "burned" even faster than Glucose.
(Presumably this is why both Corn Sugar (HFCS) and Alcohol both contribute to fatty liver disease.
In my experience (I drank a double gin in my last week but one of cruise) I was, (to quote a colleague) "slaughtered" in minutes, ("in 8 years I've never seen you so out of it"), fell asleep in the taxi on the way home, and woke up with a 2 day hangover, ravnously hungry, and aching as if I had been beaten head to toe with an iron bar.
I also weighed 2 lbs more, and a week later was still not back into ketosis.
Niche - vodka is not part of the Dukan diet in any way. As far as I can recall, the diet only mentions wine - one glass - for those times when you really, really feel you must drink, followed by a couple of days of pp, extra water, and extra walking to get it out of your system.