Some purists might question "nth"s status as a word, but since I believe language is what people speak (not what some self-appointed arbiter of style decrees), I'll agree that "nth" is a word. It is also true that "nth" isn't written with a vowel. But the question is whether you want to go with the orthography or the pronunciation.

If you go with orthography, then all you are saying is that you've found a word that isn't written with a vowel. But I think if you say "nth" outloud to yourself and listen carefully, you'll see it's really "enth".

It's been said above, but just to repeat -- there are no words in spoken English that don't have vowels. Maybe there are some foreign borrowings that don't have vowels, but I doubt there are enough English-speakers who can pronounce those words to count them as English words in their own right.

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