Hiya Simply_H!
There are no carbs in vodka and diet coke, so nothing to raise your BGs. The alcohol content keeps your liver busy, so it can't put sugar into your blood, so your BGs drop. It can be dangerous if you don't monitor and correct it properly e.g. eat some carb before bed, as alcohol can make your BG fall significantly 3 or 4 hours after you've drunk it. Monitor and correct.
Smidge