Why should being prejudiced be considered illogical? It appears factually true that humans are inferior to Vulcans, in every respect that would matter to the Vulcan Science Academy at least - so trying to pretend to view humans as equals would be disingenuous and, if the lying did not serve some strategic goal, extremely illogical.
Of course, lying is often the logical thing to do, as it promotes social cohesion in addition to serving the interests of the individual. But Vulcans lying to other Vulcans that humans are equals does not serve any obvious purpose. And the myth that Vulcans don't lie would have gotten started somewhere.
Prejudice is just judice implemented without unnecessary delay. It is illogical only when it is literally prejudice, when it is formulated before the facts are known. But in practice, prejudice refers to the accumulation of experience instead. Although many are ready to substitute hearsay for experience, which again leads to illogic.
As for racism, its arguments tend to be factual and logical. Its implementation is the aspect that is not. But I can't see a downside for Vulcans implementing racism, either. Or an upside for them abstaining from it, rather.
Timo Saloniemi
The logic is based on a faulty premise. Therefore, the conclusion is false. Six billion Vulcans are superior to six billion humans? They're physically stronger on the whole. But where's the evidence that they are superior in other ways? What have they accomplished that humans have not? They're past is even as distastively violent as the human past. They were admittedly an even more violent race, and saved themselves only through conforming to a social norm.
Vulcans perpetuating the sterotype that humans are inferior only reinforces their culture. To them, humans are loud, obnoxious, hedonistic, and fly in the face of intellect. They are trivial. And humans are this way because they let their emotions lead. Vulcans selectively forget that emotion guided the paintings of Monet or symphonies of Mozart. Emotion can be a tool for good.
McCoy Prime once said to Spock Prime that he wouldn't know what to do with a genuinely warm feeling if he had one. For Vulcans, it's about suppression. For humans, it's about expression. The society that stresses suppression has to constantly rationalize why that is better. They do it to the point where they develop a superiority complex that defies eason. Can't have too many Vulcans growing up to be Sybok. It would upset the order of things.
Vulcans profess a singular ideology that must be followed. Humans don't. The human race succeeds by stressing individual freedoms and liberties over social order and logic. Both races stand as examples of how each can work. The odd thing is the idea of IDIC fits far better into 23rd century human society than it does Vulcan.
Spock Prime learned first-hand that his people were racist against humans. He grew to appreciate the power of instinct (Kirk) and emotion (McCoy) when used for a just cause. He began to realize it probably wasn't logical to believe in only one way: the Vulcan way.