wow, so many people seem so confused as to why mixing red bull and vodka can be bad. Its mixing a stimulant (caffeine) with a depresant (alcohol). Now, fundamentally it is no different that having a rum and Coke (or any alcohol mixed with a caffinated beverage).
The problems come from the fact that Red Bull (US version) has a lot of stimulants (way more than just a coke) and most of the time that its mixed, it is mixed with more than a shot of vodka. So 1 Red Bull and Vodka can be equivalent to a few (couple) rum/cokes.
For all you little kiddies out there, Red Bull and vodka is nothing to be overly concerned about. You just have to be rational about how much you have (otherwise no sleep for you) and realize that both alcohol and caffiene are dehydrants. Therefore, drinking Red Bull and vodka will cause you to become dehydrated fairly quickly (and when drinking this means hangovers). While yes there is concern about mixing stimulants and depressants and the associated problems that you heart can have, you are not very likely to get to those levels drinking Red Bull and vodka.
life is too short to have any regrets