This study examines phonetic variation in the acoustic properties of the French /i,e,a,o,u/ sub-system as a function of vowel duration and speech style in order to better understand the interplay between these two well-known factors of vowel reduction. Over 1000k vowels extracted from three large corpora of continuous French including read speech (BREF), partly scripted journalistic speech (ESTER) and casual conversations (NCCFr) were split into duration classes (short, medium, long). Six metrics based on F1 & F2 frequencies were used to capture variation in multiple dimensions. Style and duration show comparable effects in terms of reduction in the acoustic working space (overall and in F1 or F2 dimensions), centralization of vowel categories and variability within vowel categories. However, interesting differences are found in terms of overlap between vowel categories: more neutralization of vowel contrasts independent from vowel duration is found in casual conversational speech.