As much as three quarters of the modern Vietnamese lexicon is of Chinese origin. The majority of these words are often assumed to have originated in much the same manner as late Sino-Korean and Sino-Japanese borrowed forms: by rote memorization of reading glosses that were acquired through limited exposure to spoken Sinitic. However, under closer scrutiny, this model fails to account for a broad range of features in the Vietnamese language. Through an examination of the intellectual, cultural, and political terms of Sino-Vietic contact from the 1st century B.C.E. through the 17th century C.E., as well as an analysis of the phonological forms of Sino-Vietnamese lexica that were transacted as a result, I formulate a new history of Sino-Vietic contact that differs sharply from the prevailing model. This new model departs from current concepts of Vietnamese linguistic history at three major points. First, rather than limited exposure to Sinitic language, Sino-Vietnamese phonological forms suggest that pervasive and sustained bilingual contact obtained over most of the first millennium, between the immediate ancestor of modern Vietnamese on one hand, and a local variety of Middle Chinese rooted in the river plains of northern Vietnam on the other. This requires the existence of a thriving, Sinitic speaking population in the regions of northern Vietnam that flourished over the course of the first millennium. Chapters 2, 4, and 6 are devoted to the three primary chronological layers of Sinitic vocabulary in modern Vietnamese, two of which resulted from the increasing bilingualism of this ethnolinguistically complex society. Second, based on new data from fieldwork I conducted in 2009-2010, I claim that the closest living relatives of modern Vietnamese-the so-called M!"ng varieties-in fact do not constitute a linguistic subgroup of their own, but represent distinct languages as distantly related to each other as they are to Vietnamese. As discussed in chapter 5, this model of speciation bears significant consequences for our understanding of the life and death of Sinitic language in the region. Third, I argue that the emergence of a vernacular literary tradition in Vietnam, in the form of the logographic script called Ch! Nôm, was propelled by a desire to fuse local forms with the prevailing cosmopolitan mode (i.e. Literary Sinitic), and to synthetically reproduce the kind of diglossic social architecture that had developed in medieval China over the course of the Sui and Tang dynasties. Chapter 3 examines the roots of this Sinitic diglossia in the 7th century production of the immensely influential rime dictionary called the Qieyun, while Chapter 7 investigates the rise of a vernacular literary tradition in Vietnam over the course of the second millennium, and shows how the elevation of vernacular language was justified by re-imagining it-not as a vulgar copy of Literary Sinitic, nor as a rising competitor-but simply as an extension of the domesticative and civilizing technology of Sinitic writing. By examining both the cultural and structural dimensions of language history, this dissertation provides unified account of the evolution of Vietnamese under various Sinitic influences which redefines our current understanding of Sino-Vietic contact over the last two millennia.