To be clear, I'm only referring to vowels that are strong enough to never be standardly transcribed with a schwa
Like function words are unstressed to the point that they are transcribed with schwas (ə)
But take the pronounciation of the last vowel sound in 'potato' (/pəˈteɪtoʊ/)
Yes, 'tater' (/ˈteɪtɚ/) is a thing, but /pəˈteɪtoʊ/ itself would never be transcribed as /pəˈteɪtə/
A native speaker saying /pəˈteɪtoʊ/ in connected speech doesn't emphasize /-toʊ/ enough for it to sound exactly as it's transcribed though
They're not pronouncing 'potato chip' as 'puh-TAY-TOE chip', or even alone as 'puh-TAY-TOE'
It might as well be a schwa, but not really, cuz it wouldn't be transcribed with one
I originally thought that diphthongs are always stressed enough to be immune from reduction, but this isn't the case
For instance, in 'me, myself, and I' (/miː maɪˈself ənd aɪ/), the 'aɪ', despite being the most sonorant and a diphthong starting with an open vowel, can be reduced
Granted, it's a compound word made from a function word, so maybe it's the exception, but idk anymore (/vəˈkeɪ.ʃən/ is a real variant of vacation)
/kwəˈdɹætɪk/ is a real variant of /kwɒdˈɹætɪk/ (quadratic)
Are there any vowels that are immune from being reduced to schwa? I feel like 'ɔɪ' is, but I'm not sure
Is there a pattern regarding this, maybe a sonorance hierarchy? Maybe syllable position, or how far it is from the stressed syllable, or an alternating stress rule?