I have tried the Crimson Cow from FreedomSmokeUSA - IMO it just tasted like cherry (but i think my high nic level may have affected this)
I have also tried Top Vapor's Mad Torro from MadVapors - It tasted pretty much exactly like the Crimson Cow (was also a high nic level though)
the Best RedBull flavour I have found so far, is from a company called TTZig.
I have ordered it twice, I liked it so much... the second time it didn't seem AS spot on, but i think that was caused by a change in hardware. but the scent alone is dead nuts RedBull.
I'm also interested if anyone else has found any other 'believable' RedBull ejuices.