I'm not sure if this is a useful question or not. But I was surprised to notice まさか (masaka) was using a single vowel (a) for the whole word. The surprise came from the connection to "bakana" (バカな) which apparently is two words, so never mind that.
I set out to discover more of these single-vowel words in Japanese, but I don't really know Japanese or how to search for such things. So I found a list of words and analyzed them here https://colab.research.google.com/drive/194vb5Ajw4mMlUcb1HqF4YF2aP1LjeVlS
This prints out:
Found 22 single column words あなた うつむく からだ くぐる さまざま せめて そのもの そもそも そろそろ だから つくる つづく ところ どころ なかなか ながら ふつう まさか もともと ものの よほど わざわざ
Which I guess is a cute tongue-twister. The question is - does this class of words have a name? Are these words more poetic in a way? Are there other, longer, words like this that I missed?