Related to this, I would like to share a near-personal experience.
I was an observer (who stuck to beer) at a party where two guys had the brilliant idea of trying this. Additionally, it was only the two of them and 4 cans of 250ml each of Red Bull as well as the same amount of vodka. The resulting concoction was 2l of an off-yellow brightly colored liquid, which they proceeded to gulp down like thirsty chickens.
(A lot of pseudo-medical nonsense deleted after someone with more knowledge than me bothered to enlighten me. Thank you.)
Almost instantly after the effects hit, both of the guys started vomiting violently. Luckily we were all outside at the time. At the same time a sudden loss of coordination had them rolling around on the grass for quite a while. This phase didn't last very long. One of the guys spent most of the night periodically falling over into his mother's flowers while sitting in a lawn chair. He was the lucky one. The other one spent most of the night hugging a toilet seat asleep, periodically waking up to vomit his guts out. A few hours later, and in the morning, he was vomiting a yellow sludge we assumed to be stomach acid. When the vomiting had calmed down, we carried both of them to the nearest couch/bed, where they found each other in the morning, with one's arm lovingly holding the other close, and no recollection whatsoever of the previous night.
As far as I know neither has bothered with Red Bull and vodka since.