[bradrn]

> as a native speaker, I can't even decide if that should be "first vowel in these is" or "first vowels in these are" or what I actually wrote above which is what seems more natural to me, although immediately stood out to me as grammatically inconsistent when I re-read it after posting...

I would say that the former (“first vowel in these is”) is ‘more correct’, but it sounds weird because it contains the plural “these” immediately before the singular “is”. What you actually wrote is inconsistent strictly speaking, but it feels better because the verb agrees with the immediately preceding word. (This kind of thing is rather common in languages with agreement.)