Aliquando is probably better than interdum, though both are acceptable. Aliquando shows up more frequently in contrasts with words like semper and numquam, and I suppose the point of the quote is that the first five are always vowels, while the last is only a vowel sometimes. I’m reminded of a quote from Tacitus’ Agricola: Haud semper errat fama; aliquando et elegit - “Public opinion (fama)doesn’t always err, sometimes even it chooses correctly.”
A, E, I, O, U, et aliquando Y