This dissertation studies the synchronic phonetics of the tonation systems of five Vietic (Austroasiatic) varieties, Arem (ISO: aem), Rục (ISO: scb), and three North-Central Vietnamese (ISO: vie) dialects, Cổ Định (Thanh Hoá province), Nghi Ân (Nghệ An province), and Diêm Điền (Quảng Bình province) with the aim of improving reconstructions of Proto-Vietic tonation. Tonation is a cover term for tone and register, two phonological contrasts frequently encountered in many East and Southeast Asian languages. These two contrasts can be realized on the rhyme by differences in f0, phonation type, and duration, but also, especially in the case of register, by differences in vowel quality and by modulations of closure duration and voice onset time in onset consonants. Tone usually derives from the loss of laryngeal codas (tonogenesis), while register typically originates from the loss of voicing contrast in onset obstruents (registrogenesis). Onset devoicing can also lead to the formation of two-tone systems or double the number of tones in tonal languages. Different hypotheses have been advanced to explain the phonetic underpinnings of tone and register development, their shared phonetic properties, and intersecting segmental origins, but experimental evidence is needed to validate these hypotheses. The results show that Arem contrasts high and low registers realized by means of a breathier phonation and more closed vowels in the low register. However, Proto-Vietic final glottals, *-ʔ, *-h, and glottalized sonorant codas are still preserved in the language, and their effects on duration, f0, and phonation of preceding vowels are too variable across registers and speakers to consider Arem as tonal. The study of the tonation system of Rục shows that it has four tones in open and sonorant-ending syllables. These tones can be grouped into two high-register tones and two low-register tones that are distinguished by consistent differences in f0, phonation type, vowel quality, and to a lesser extent, vowel duration and VOT. The final fricative *-h is still preserved in Rục, and syllables ending in this coda exhibit no f0 difference, but they feature all other register properties. Hence, Rục has a hybrid system of tone and register. The results of the perception experiment reveal that Rục participants use the same phonetic cues in their perception as in their production of tonation, with roughly the same prominence for each cue. I then turn to the three North-Central Vietnamese dialects and show that they have different numbers of tones and make use of different phonetic properties to realize these tones. Different post-tonogenetic changes also led to distinct tone mergers in these dialects. Traces of register-conditioned variation in phonation type and vowel quality are limited in these varieties, but the low-register tone A2 in Diêm Điền was found to preserve breathy phonation, which is likely to be a remnant of the low register in Proto-Việt-Mường. Vowel quality was found to be conditioned by the tense phonation of tones in the low f0 range in Cổ Định and Nghi Ân, but I argue that this is not a remnant of register, a conclusion strengthened by an investigation showing that there is little evidence for register-conditioned vowel changes from Proto-Vietic to modern Northern Vietnamese. Since there is either a register contrast or remnants of register in all branches of Vietic, I propose to reconstruct a register contrast in Proto-Vietic, against previous proposals postulating that it had a voicing contrast. This register contrast evolved differently in different daughter languages and interacted with various patterns of retention of laryngeal codas, resulting in diverse tonation systems in Vietic languages. By combining insights on the typology of tonogenetic effects gained from my investigation of Rục and Arem glottal codas and on evidence from correspondences between Chinese and Sino-Vietnamese tones, I further propose that tonogenesis from codas is likely to have happened in Proto-Việt-Mường from the 9th to 12th century CE, which is later than previous proposals.