Vowels definitely require tongue movement! Leave your mouth closed and whisper "a-e-i-o-u" and you can feel your tongue moving.

Vowels change the shape of your mouth's cavity to change which frequencies to emphasize ("formants") and we hear the differences in vowels by analyzing those formant patterns.

There's subtle changes in internal speech muscles as well often for those kinds of movements, and where it does seem ambiguous language context is usually enough to distinguish