I recently recalled a debate regarding vocal harmony in Resian dialect/language that de Courtenay saw as evidence of Turkic influence in formation of that dialect.
However, further digging revealed that Slovene language scholars found similar vowel assimilation across syllables in most Slovene dialects as the first step in the so-called Modern Vowel Reduction. As modern Slovene orthography doesn't reflex this vowel reduction, and neither does pronunciation of the language standard, this isn't apparent, unless you acquaint yourself with actual spoken dialects.
So, I'm back with more examples to support this counterargument, with words I found in my own dialect (which is somewhere in between Ljubljana and Upper Carniolan dialect group). I've mostly found examples of tensing, where following vowel causes the preceding stressed vowel to be pronounced as tenser, more closed.
daleč ("far away"):
derived form OS *dalekъ. In Standard Slovene it's pronounced /'da.lɛtʃ/, I pronounce it as /'dɛ.lətʃ/ — apparently first step being assimilation of /a/ into /ɛ/, followed by reduction of unstressed /ɛ/ into /ə/.
jesen ("ash tree"):
derived from OS *jasenъ (it is cognate to Russian ясень), irregularly derived in Standard Slovene with this assimilation of /a/ into /ɛ/. In Standard Slovene it's pronounced /'jɛ.sɛn/, I pronounce it as /'jɛ.sən/ because of following reduction.
kakor ("as", "like"):
derived from OS *kako-že ("how-that"); rhotacism occured in early stage of development, already attested in Freising Manuscripts, late 10th century. In Standard Slovene it's pronounced /'ka.kɔr/, I pronounce it as /'ko.kər/ — three steps, first assimilation of /a/ into /ɔ/, then reduction of unstressed /ɔ/ into /ə/, and tensing of stressed /ɔ/ between two velars into /o/, which is regular development of Slovene pronunciation.
There's also one example of a vowel change in a prefix po- which consistently occurs before stressed /u/ and /i/ in my dialect:
poslušati ("to listen"):
In Standard Slovene it's pronounced /pɔ.'slu.ʃa.ti/, I pronounce it as /pu.'slu.ʃət/. Apparently, in this case, /ɔ/ assimilates into stressed /u/ from base word root in next syllable. The last two vowels are victims of vowel reduction, first one is transformed into /ə/, the second one disappears.
popisati ("to write on", "to make an inventory"):
Standard Slovene /pɔ.'pi.sa.ti/, I pronounce it as /pu.'pi.sat/. Again, same assimilation in prefix, but the /a/ doesn't get reduced, probably because the accent is a bit more fluid in this case (other words with same word root can be accented on that /a/).
povedati ("to tell"):
Standard Slovene /pɔ.'ʋe.da.ti/, I pronounce it as /pɔ.'ʋe.dət/. No assimilation in prefix, /e/ isn't tense enough to cause it.
However, this primitive vowel harmony system isn't really productive and only appears in specific cases. Unlike Resian, which instead of going through Modern Vowel Reduction, developed a vowel harmony based on assimilation of tenseness and breathiness (breathy vowels appeared in Resian instead of vowel reduction) of vowels.
So, now that I've described a dialectal feature; let's start a debate on this topic, shall we?
Besides Altaic vowel harmony, there is also one other well known vowel assimilation system: the Germanic Umlaut. But, I'm not interested in those cases. I'd like to hear if you perhaps know of a dialect that does similar wacky stuff with vowels. I'd imagine this happens in dialects somewhat regularly, but rarely carries over onto entire languages in language development. Because as soon as a language starts cramming words together and reducing vowels, some of the quality of those lost vowels should be passed onto its surrounding, to preserve distinctions between words.