Sedang itself has 24 pure vowels: 7 vowel qualities, all of which may be plain (ex. [a]), nasalized ([ã]), and creaky ([a̰]), and three of which -- /i a o/ -- may be both nasal and creaky ([ã̰]). While it doesn't have the length distinctions of other North Bahnaric languages, it does have more diphthongs, for somewhere between 33 and 55 vowel sounds altogether. Because of this Sedang is sometimes claimed to have the largest vowel inventory in the world. Note however that other Bahnaric languages have larger numbers of vowel qualities (Bahnar, for example, has 9) in addition to phonemic vowel length, so the record holder depends closely on how the languages are described and distinct vowels are defined.
References
Smith, Kenneth D. 1967. "Sedang dialects." Bulletin de la Société des Etudes Indochinoises 42: 195-255.
Smith, Kenneth D. 1968. "Laryngealization and de-laryngealization in Sedang phonemics." Linguistics 38: 52-69.
Smith, Kenneth D. 1969. Sedang ethnodialects. Anthropological Linguistics 11(5): 143-47.
Smith, Kenneth D. 1973. More on Sedang ethnodialects. Mon-Khmer Studies 4: 43-51.