Tetsó ˛t'ıné is a dialect of Dëne Su ˛łıné (ISO: CHP) spoken in Canada's Northwest Territories. The verb system of Tetsó ˛t'ıné has only recently been described (Jaker and Cardinal 2020); this paper is the first to propose an analysis of the distribution of long and short vowels in Tetsó ˛t'ıné prefixes. In Tetsó ˛t'ıné, all long vowels in prefixes are derived from intervocalic consonant deletion, although not all cases of intervocalic consonant deletion result in a long vowel. Whether or not deletion of an intervocalic consonant results in a long or short vowel depends on a combination of two factors: the consonant that was deleted, and the morphological level to which the preceding prefix belongs. In this paper, I propose that the basic generalization about prefix vowel length can be stated in terms of SYSTEMATIC UNDERSPECIFICATION (Kiparsky 1993). I claim that prefix vowels, unlike stem vowels, have zero moras underlyingly, and only acquire a mora after passing through at least one level of the phonology. This analysis predicts that prefix vowel length ought to be subject to a Derived Environment Effect (DEE), for which there is indeed evidence. The pattern of mora insertion in Tetsó ˛t'ıné prefix vowels is thus an example of the interleaving of phonology and morphology, and illustrates how phonological behaviour can be to some extent predicted based on morphological structure.