What word (which doesn't contain the letter y) only has one vowel even though it has nine letters?
Some words with the desired property, but containing more than nine letters are:
Strengthlessnesses (eighteen letters)
Defenselessnesses (seventeen letters)
Defenselessness (fifteen letters)
Degenerescence (fourteen letters).
Degenerescent, disinhibiting, effervescence, handcraftsman,
kinnikinnicks, philistinisms, primitivistic, retelemetered, whipstitching( thirteen letters).