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The answer is that alcohol lowers the liver's ability to release emergency sugars that all people store in their livers in case they haven't eaten in a long time and glucose levels drop (which can happen to non-diabetics as well as to diabetics, but which can be life-threatening to the type 1 diabetic). So one of the dangers that type 1 people need to be on the lookout for is the danger of getting hypoglycemic and passing out from low glucose after drinking a lot. Most people would assume that this person is just acting funny or is semi-conscious because they are drunk, but the reality is that this could be a life-threatening episode of hypoglycemia, which can kill the type 1 diabetic if that person doesn't get some carbohydrates into their system.
As for beer vs. vodka...
Straight vodka has zero carbs, so has no effect on glucose levels other than the effect of lowering the body's ability to pull in emergency stores of sugar from the liver if glucose levels drop too low. Beers, on the other hand, do have some carbohydrates, and they need to be figured in when deciding how much insulin to take. Different beers have different amounts of carbohydrates in them, depending on the brand. A chart at the following website shows a comparison of carbohydrate counts per 12 ounces of various beers. Obviously, the diabetic person needs to make sure that he or she has enough insulin working to cover the carbs.
http://www.bellaonline.com/articles/art14302.asp