Purpose: The impact of clear speech, increased vocal intensity, and rate reduction on acoustic characteristics of vowels was compared in speakers with Parkinson's disease (PD), speakers with multiple sclerosis (MS), and healthy controls. Method: Speakers read sentences in habitual, clear, loud, and slow conditions. Variations in clarity, intensity, and rate were stimulated using magnitude production. Formant frequency values for peripheral and nonperipheral vowels were obtained at 20%, 50%, and 80% of vowel duration to derive static and dynamic acoustic measures. Intensity and duration measures were obtained. Results: Rate was maximally reduced in the slow condition, and vocal intensity was maximized in the loud condition. The clear condition also yielded a reduced articulatory rate and increased intensity, although less than for the slow or loud conditions. Overall, the clear condition had the most consistent impact on vowel spectral characteristics. Spectral and temporal distinctiveness for peripheral-nonperipheral vowel pairs was largely similar across conditions. Conclusions: Clear speech maximized peripheral and nonperipheral vowel space areas for speakers with PD and MS while also reducing rate and increasing vocal intensity. These results suggest that a speech style focused on increasing articulatory amplitude yields the most robust changes in vowel segmental articulation.