Hearing-impaired speakers show changes in vowel production and formant pitch and variability, as well as more cases of overlapping between vowels and more restricted formant space, than hearing speakers; consequently their speech is less intelligible. The purposes of this paper were to determine the differences in vowel formant values between 32 hearing speakers, 14 severely hearing-impaired speakers, and 25 profoundly hearing impaired speakers, and to investigate the influence of perceptual constraints on the contrastiveness of spoken vowels in speakers with hearing loss, as these underline the importance of good phonation, articulation, and resonance in speech production. Several differences in formant values were confirmed with Anova-Welch tests, except for the F1 of open /e/ and /a/, the most open and loud vowels in Slovene. In addition, discriminant analysis showed real differences in vowel production (97.1% classification success), thus making it possible to differentiate between groups of hearing and hearing-impaired speakers based on this one function of vowel production. The hypothesis was confirmed that vowel production in hearing-impaired individuals is different from that in hearing individuals. The results suggest that speech rehabilitation should consider specific tasks and training for vowel production, including neuromuscular oral control and self-monitoring, to improve speech intelligibility. The results are also useful for those who are in contact with people with hearing impairment, such as teachers, audiologists, speech and language pathologists, nurses, and researchers in speech and hearing sciences. (Contains 8 tables and 5 figures.)