In this dissertation, I provide a new analysis of the Vietnamese vowel system as a system with fourteen monophthongs and nineteen diphthongs based on phonetic and phonological data. I propose that these Vietnamese contour vowels - /ie/, /ɯɤ/ and /uo/ - should be grouped with these eleven monophthongs /i e ɛ a ɐ ʌ ɤ ɯ u o ɔ/ based on their similarities in phonetic and phonological behaviors. The phonetic characteristics of these vowels are studied acoustically using normalized and scaled acoustic values of 13,925 tokens, spoken by female Hanoian speakers from my speech corpus, "The Vietnamese Speech Corpus". Phonetic analysis shows that the eleven monophthongs and three contour vowels are similar in terms of formant frequency targets, formant dynamic trajectories, and duration. Phonologically, monophthongs and contour vowel can be rhymed with each other in poems, and the two elements within each contour vowel should be analyzed as two halves of one root node in the syllable structure. In chapters 1 and 2, I give the current analysis of the Vietnamese sound system, review different approaches to the acoustic features of vowels, and the phonemic status of diphthongs. In chapter 3, I give a detailed description of the Vietnamese Speech Corpus. In chapter 4, I show the difference in formant targets between monophthongs and glides, as well as the importance of duration in distinguishing vowels in Vietnamese. I also give evidence for the differences in duration between the diphthongs and the monophthongs-and-contour-vowels group. In chapter 5, I analyze the natural class of monophthongs and contour vowels in terms of feature geometry and give evidence from Vietnamese phonological processes to support the analysis of contour vowels as being in the same natural class as monophthongs. I also re-analyze Vietnamese triphthongs as diphthongs in this chapter. Finally, in chapter 6, I summarize the similarities and differences across the monophthongs, contour vowels, and diphthongs, and suggest possible future studies to test this hypothesis of the Vietnamese vowel system.