When singing in languages like Italian or Latin (which don't run to diphthongs the way English does), I sometimes find it hard to sing pure vowels, particularly when holding a note for a while. Should I attack this as an "ear" problem (and I need to listen to more music in these languages), or is there some vocal technique I can apply that will help?
Interesting question! The only tip that springs to mind is to think of constantly renewing the vowel, so rather than thinking "I'm singing a long oo, here I go, ooooooooooooooooo" it can be helpful to think of lots of tiny, pure oo's strung together. ("oo-oo-oo-oo-oo-oo-oo-oo")
You may also want to be aware of what your lips and tongue are doing during a long note, as if they change shape they will necessarily affect the vowel sound.
It may be helpful to take out all of the consonants and sing through a song in Latin or Italian with only the vowels. For instance Dona nobis pacem would become oa oi ae. That will allow you to focus more specifically on the vowel sounds without the consonants getting in the way or encouraging you to run to dipthongs.
The easiest way to hold a pure vowel sound is to maintain mouth position. Take, for instance, the 'O' sound of wrote. It is easy to close the jaw slightly to form the 't' and create a diphthong of the 'O' as in the phonetic o-oot. Instead keep the jaw relaxed on the O sound and instead of closing the mouth, reach up with the tongue to softly lick the t sound from the top of the mouth. Most deviations from pure vowel sounds occur when encountering the following consonant sound. Emphasis on consonants can be avoided - even when we should have a diphthong. Try the word wild --- most people triphthong this to ooWah-eeeee-ald because of jaw position. Instead, sing "hhhhWahhhhhhhhh- eeld" with very little movement of the jaw and just a small eeld with the tongue against the roof of the mouth. Start breathing before engaging the vocal chords to prevent a loud start that falls back in volume. We need not have the ooWah begin words that begin with W.
IMO, this is practice, practice, practice. Go over the song phonetically, and speak each word on a single tone or in a speaking voice, properly pronouncing each vowel. Always remember that note transitions are where your dipthongs and consonants should be placed, and it should happen as fast as still sounds natural.
Now, that said, you imply that "pure" vowels equal "Latin" vowels. This is not necessarily the case; you can sing unelisioned vowels, and be consistent across an entire choir as to the color of each vowel, without the vowels having to be "ah", "eh", "ee", "aw", "ooh". The Latin pronunciation is simply an easy, natural, dialect-neutral set of vowels that works for classical music in most languages.
There are many techniques, and the answers so far are great, but I'll add another suggestion that works great for some folks:
Modify the words mentally. Instead of singing a-bout (which can lead to funny dipthongs), think of it phonetically as uh-bah-oot. The vowel you're holding for a long time is close to "ah", and the "oo" from the u is more part of the final t sound than part of the previous vowel.
Eventually, you learn to make these kinds of modifications automatically.
It is all about becoming conscious, with each syllable you sing, of the different positions and shapes of all the parts of your vocal apparatus: lips, tongue, soft pallate, and the placement of the tone in your head. And whether or not these positions are moving or changing during the production of the vowel. (if they move or change while you make one vowel sound, this is what creates a dipthong.)
The best way to learn this is to take lessons from a voice teacher or sing with a choir director who gives you feedback on your vowel production and shows you what you are doing wrong.
The good news is that, with continued practice of good habits, this meticulous and constant self-examination and self-awareness of the changing positions of your lips and tongue, etc., eventually become second-nature and you no longer have to think about them constantly as you sing.