Find my polyphthongs!
Python 2, 102 97 bytes

thanks to @JonathanFrech for -5 bytes



w=input();l=range(len(w)+1)
print{w[a:b]for a in l for b in l if b-a&gt;1&lt;set(w[a:b])&lt;=set('aeiou')}


Try it online!

lowercase I/O