As far as everyday English words are concerned the most consecutive vowels to be found in a single word is 5. This occurs in two words: queueing and cooeeing. However there is term used in medieval music: euouae, which has 6. This is quite a contrived word, in fact it is an abbreviation. Wikipedia describes it thus: Euouae (/juːˈuːiː/) is an abbreviation used in Latin psalters and other liturgical books to show the distribution of syllables in the differentia or variable melodic endings of the standard Psalm tones of Gregorian chant. It derives from the vowels in the words “saeculorum Amen” of the lesser doxology or Gloria Patri, which ends with the phrase In saecula saeculorum, Amen. So there you have it.