Data collected on the 17 languages spoken in the Banks and Torres Islands (northern Vanuatu) reveal strikingly diverse vowel systems, differing both in the quality and the quantity of their phonemes. Except for Mota, which still perpetuates the five vowels of Proto-Oceanic, the languages of this area have historically increased their inventories to as many as 13 and even 16 vowels. The aim of this paper is to track the systematic correspondences between modern languages and their common ancestor, and to reconstruct the processes that led to the present-day phonemic diversity. The phonemicization of new vowels, including diphthongs and long vowels, is shown to result from stress-induced vowel reduction and metaphony. This general process of "vowel hybridization" yielded results that differed from one language to another, and sometimes within the same language. After describing and classifying the various patterns of sound changes attested, this paper shows how a proper understanding of vowel hybridization proves indispensable for the reconstruction of both the lexicon and the historical morphology of these northern Vanuatu languages.