Ultimate attainment in foreign-language sound learning is addressed via vowel production accuracy in English spoken by advanced Czech EFL learners. English FLEECE–KIT, DRESS–TRAP, and GOOSE–FOOT contrasts are examined in terms of length, height, and backness. Our data show that, while being constrained by phonemic category assimilation (new vowel height distinctions are not created), the learners’ interlanguage combines phonological parsimony (reusing L1 length feature to contrast L2 vowels) with phonetic flexibility (within-category shifts reflecting L1–L2 phonetic dissimilarity). Although achieving nativelike phonological competence may not be possible learners who acquire L2 in the prevailingly L1 environment, the Czech learners’ implementations of English vowels revealed their ability to adjust for phonetic detail of L2 sounds.