I think its due to the reccomendation that you shouldn't drink more than 3 cans of RB in a night, as it may cause heart palpitations/tachycardia etc, as well as similar effects to the abuse of amphetamines. Due to the situation in which you drink VRB, you are likely to consume large levels of caffeine in a short period. The vodka is just incidentally harmul too in large doses. and consuming large amounts of caffeine in addition to alcohol doesn't affect the effects of alcohol, just makes you think you are more alert, and may make you more voulnerable.