The present investigation examined the extent to which asymmetries in vowel perception derive from a sensitivity to focalization (formant proximity), stimulus prototypicality, or both. English-speaking adults identified, rated, and discriminated a vowel series that spanned a less-focal/prototypic English /u/ and a more-focal/prototypic French /u/ exemplar. Discrimination pairs included one-step, two-step, and three-step intervals along the series. Asymmetries predicted by both focalization and prototype effects emerged when discrimination step-size was varied. The findings indicate that both generic/universal and language-specific biases shape vowel perception in adults; the latter are challenging to isolate without well-controlled stimuli and appropriately scaled discrimination tasks.