A useful compromise (given how high the alcohol content of spirits is) would be to have your first diet coke with vodka, and then have refills without (while of course telling others that there is still vodka in it). new year resolutions and all that
Just be carful with the mixers you get. Some bar staff don't give a monkeys what they serve you and with a Vodka it's hard to tell fit it's diet of not. You can test with your drink with your blood glucose meter.
no alcohol is safe for a diabetic ( see my post on alcohol and diabetes ) but beyond that the only thing that actually matters is personal taste, I used to drink but serious alcohol related complications started and have left me permanently disabled with neuropathy so haven't drank for last 17 years apart from the odd one at home.