Words consisting entirely of vowels include AA (a type of lava), IAO, OII, EUOUAE, OO, I, O, A, and IO, which is an interjection in Chambers in addition to being one of the moons of Jupiter. All-vowel words in the OSPD3 are AA, AE, AI, EAU, OE. OO means "wool" (Scottish). The OED has the interjection AIEEE. AI is the three-toed sloth in Chambers and a Biblical place name and a Russian river. Other words consisting only of vowels are UOIAUAI (the name of a language in ParŽ State, Brazil) [Dmitri Borgmann, in Charles Bombaugh's Oddities and Curiosities of Words and Literature, ed. Martin Gardner). The Samoan word UAEA means "wire" and is actually the English word spelled phonetically [Rémy Viredaz]. AIOUEA is a genus of the laurel famliy (Lauracea) [Charles Turner]. The scientific name of the roseate spoonbill is AJAIA AJAIA (or AJAIA AJAJA); however, the J's would have been I's in the original Latin spelling, so that the words would consist entirely of vowels also.
Audubon L. Bakewell IV provided a list of some all-vowel Hawaiian names for birds which are used in English: AO - Puffinus puffinus (Manx shearwater), UAU - Pterodroma phaeopygia (Dark-rumped Shearwater), OEOE - Oceanodroma castro (Harcourt's Storm-petrel), IO - Buteo solitarius (Hawaiian Hawk), OOAA - Moho bracatus, OU - Psittirostra psittacea.
Allowing proper nouns, some words consisting only of vowels include AIEA (a city in Hawaii), EIAO (one of the Marquesas Islands), EA (a town in the Basque section of Spain), AEAEA (the legendary home of Circe in the Tyrrhenian Sea), AAUAUA (a river in Brasil), II (a place in Finland 40 km north of Oulu), and OUEOI (an ancient Tuscan city in Etruria). IO is the only name on any hurricane name list which consists entirely of vowels. It is on the Central Pacific hurricane name list [Brett Brunner, Juozas Rimas, Stuart Kidd, Pertti Malo]. Dmitri Borgmann gives OEAEI (the name of the wife of Ra-amen, the spondist of Pthah) [An Archaic Dictionary, From the Egyptian, Assyrian, and Etruscan Monuments and Papyri, by William R. Cooper, London, 1876]. OUADI EL AOUAOUIYE is a location in Syria, AOUEOUA in Mauritania, and AOUEOUE in Togo
Allowing Y , some words consisting only of vowels include AYE, AYU, EYE, IYO, EYEY, and YAYA