Here are some references to relevant studies about Red Bull improving vigilance:

1. Mets MA, Ketzer S, Blom C, van Gerven MH, van Willigenburg GM, Olivier B, Verster JC. Positive effects of Red Bull Energy Drink on driving performance during prolonged driving. Psychopharmacology 2010, DOI 10.1007/s00213-010-2078-2
2. Barthel T. et al., Readiness potential in different states of physical activation and after ingestion of taurine and/or caffeine containing drinks, Amino Acids 20, 1, 63-73 (2001)
3. Horne J.A., Reyner L.A., Beneficial effects of an “energy drink” given to sleepy drivers, Amino Acids 20, 1, 83-89 (2001)
4. Reyner LA and Horne JA, Efficacy of a ‘functional energy drink’ in counteracting driver sleepiness, Physiology & Behaviour 75, 331 - 335 (2002)
5. Seidl R. et al, A taurine- and caffeine-containing drink stimulates cognitive performance and well-being, Amino Acids 19, 3/4, 635-642 (2000)
6. Alford C. et al., The Effects of Red Bull Energy Drink on Human Performance and Mood, Amino Acids 21, 2, 139 – 150 (2001)