My previous example of how to pronounce 'button' varies depending upon where you're from...but is still recognized by the most English speakers. Some folks pronounce it but-ton, others but'n.
Water can be pronounce in a variety of ways. Actually the list is endless...I'm surprised you're arguing the opposite. Compared to Spanish, or any number of languages where the vowels are very specific (disregarding the Castilian ascent), English is very forgiving. Bringing up Khmer's letters was to show that the vowel sounds have been formalized in that language, where English allows for much more variance.
Listen to any non-native English speaker, they can be all over the place and still understood.
This is a distinction in consonants! Different realizations of the /t/ phoneme. The whole claim that started this thread was that English expresses information almost exclusively in consonants, not vowels. Your example directly contradicts that very claim.
Anyway, English is _far_ from the only language where different dialects have a different set of vowels. Quebec French for example has completely different ones from French French.
Again, we are not talking about “letters” or the writing system here. Yes, there are many distinct vowels that are written the same, and conversely many different ways of writing the same vowel, but that’s irrelevant to the discussion of spoken language.
And, yes, I know what a tonal language is, and you’re right that English isn’t one. It’s hardly unique or special in that. Most languages aren’t tonal.