It's way too easy when we assume that input is unique

Maybe easy for you, but not for everyone.

There's nothing about words with more than one vowel

That's a fair point. The way the problem was originally posed to me, you would really be looking through a dictionary of English words, of which there are no instances of multiple vowel alternations across a set of words. To generalize this type of search problem to other natural languages, or non-natural inpu, one could arbitrarily define what the correct "solution" should be.