It sounds like it's true that Ukrainian vowels undergo reduction when not stressed (like in the second syllable of тридцять), similar to Russian vowels, but I think the reduction is not quite as widespread as in Russian. Is that correct? Also, is the Ukrainian letter 'о' ever reduced? The Russian vowel 'о' often undergoes this reduction: "достаточно"
There should be a difference between stressed and unstressed syllables but no reduction as far as I know. When a proper Ukrainian pronunciation is taught (I skimmed through one such course) evenness of vowel sounds is always encouraged. If you do hear some reduction in a real-life speech it's either very negligible or from a nonnative speaker.