Words that are made up of all vowels!
It may just make you clench your bowels! (sorry, not good at poetry.)
Anyways, feel free to google, I have no way of stopping you 
‘you’ comes to mind.
ah
aye
ewe
eye
oh
ow
owe
(assuming the question means vowel sounds)
Oui
You didn’t say it had to be English 
If not, I assume you’re stuck with two English words: a + i
“O” is also an English word, albeit not used much in prose.
Eau
Sometimes w?
Whew
There aren’t any Hebrew words that qualify. That may not be technically true, I forget all the details of the alphabet, but most of the vowels are unwritten.
I.O.U.
Do you consider diphthongs vowels? If not, many of the examples given don’t count. And in the case of ‘you’ and ‘whew,’ y and w(h) are consonants. Which actually begs the question: are you talking about spoken and/or written forms?
Aaaaiiiiiieee!
Then there’s Aiea, a city in Hawaii.
aa - a type of lava
ae - a Scottish variant of “A”, meaning “one”
ai - a three-toed sloth
ao - a color word, from the Japanese, including both blue and green
Au - a district in Munich, Germany
ay - or “aye”, “yes”
No way am I playing you at Scrabble.
au isn’t good in English language scrabble, so you could challenge it off the board.
Ah, but this isn’t Scrabble.
ea - a river or waterway
Ee - an island in the Cook Islands
Ei - a town in Japan
Eo- an artificial language
eu - Greek for “good” or “well”
ey - a variant of “eh”
Iowa, if W counts as a vowel
Oy.
I suspect that is Yiddish and not Hebrew.
Ia - a river in Brazil
ie - a Cyrillic letter that resembles the letter e
Ii - a town in Finland
Io - a moon of Jupiter
Iu - a Korean singer