In modern Chinese, the [vi sound—derived from Middle Chinese [v]+[i]— persists in Central Plains Mandarin and beyond, exhibiting diachronically stratified pronunciation across five regional categories in China and abroad, shaped by historical geography, warfare, and commerce. Focusing on six Zhi-category characters (味,唯,微,惟,未,维) with Yu-Wei initials and rounded vowels, this study takes Central Plains Mandarin as its primary field, drawing on the Guangyun and Zhongyuan Yinyun to trace their spatio-temporal evolution. Grounded in Dai Zhen's phonetic transformation theory, it reveals the bidirectional closed-loop mechanism of "initials driving rhyme differentiation and finals reinforcing initial assimilation," extending traditional explanatory scope. Comparing three domestic dialect types (Central Plains Mandarin, Wu, southern peripheral dialects) with two overseas materials (Central Asian Dungan Mandarin islands and Sino-Vietnamese phonology), it demonstrates that [vi] constitutes stratified remnants of different diachronic stages, with overseas materials complementing domestic dialects. This study provides empirical evidence for interdisciplinary research bridging historical phonology and dialect geography, with both academic and cultural value.