from
http://members.aol.com/gulfhigh2/words6.htmlSome long words with only one vowel which is allowed to repeat are: TATHAGATAGARBHA, TARAMASALATAS, HANDCRAFTSMAN, STRENGTHLESSNESSES, DEFENSELESSNESSES, EFFERVESCENCE, RETELEMETERED, DEGENERESCENCE, BEEKEEPER, and perhaps EYELETEERS, which is in OSW but I am unsure whether the Y is a vowel), INSTINCTIVISTIC, DISINHIBITING, KINNIKINNICKS, PHILISTINISMS, PRIMITIVISTIC, WHIPSTITCHING, MISSISSIPPI, LOXOLOPHODONTS, MONOPHTHONGS, POSTWORKSHOP, OCONOMOWOC (a town in Wisconsin), STRULDBRUGS, CURUCUCUS, DUMBSTRUCK, UNTRUTHFUL, NUMBSKULLS, MUNDUNGUS, USUFRUCT, SUSURRUS, SUCCUBUS, GLYCYLS, RHYTHMS. In addition CHRONONHOTONTHOLOGOS (in Roget’s Thesaurus, at "blusterer"), the title and hero of a 1734 burlesque tragedy by Henry Carey, is used for "anyone who delivers an inflated address" (Brewer's Phrase and Fable). In his book Language on Vacation, Dmitri Borgmann calls these words univocalics and lists SYMPHYSY as the longest Y-univocalic [Marc Broering, Dan Tilque, Stuart Kidd, Philip Bennett].
I didn't check any to see if I could find in online dictionaries