Even though the main stress is on the 2nd syllable, it should still be possible to hear the different vowels in the first syllable (pronounced with secondary stress). You have these vowels in Mandarin (Hanyu pinyin ei and ou respectively):
/veɪˈkeɪʃ(ə)n/
/voʊˈkeɪʃ(ə)n/
That said, Americans may occasionally pronounce “vacation” with a reduced vowel in the first syllable in rapid speech, but the meaning should be obvious in context.