i used to be all for redbull and vodka or any energy drinks and alcohol, up until recently when i had a bunch of really bad nights consecutively, all with very different amounts of everything every night.
one of the ways redbull works is b vitamins, which assist in metabolism, and by assisting your metabolism you are given more energy. when you mix with vodka or other alcohol the redbull speeds your metabolism, which speeds up your body's metabolism of the alcohol you have ingested as well as anything else you have ingested that has yet to be digested and metabolized, essentially getting you more drunk, faster, and on an empty stomach. this is also why you feel the crash.
another effect of redbull and energy drinks is dehydrating your body, which is also an effect of alcohol, which will increase the dehydration you feel in the morning.
now if you keep eating and drinking water throughout the night these things will not be an issue, but its pretty hard to remember to eat and drink while you are at a party, club, pub, bar, etc.
in my opinion, redbull and vodka (energy drinks and alcohol) are fine if you drink them in moderation, and have them as your primary (or only) alcoholic beverage of the night.
not to mention, they cost so much to buy when you are out.