Some dialects of Vallo di Diano, in southern Campania (Italy), display variation in final unstressed position between four full final vowels -[i e a o] and a single centralized final vowel -[@]. In this paper we will discuss experimental data from three varieties of that area, those of Polla, S. Pietro al Tanagro and Sanza, and argue that variation in the quality of final unstressed vowels is best interpreted in terms of an interaction between a basilectal, Tuscan-like final vowel system -/i e a o/ and the Neapolitan-based koine variety in which all final unstressed vowels merged to -/@/. The choice of this interpretation over conceivable alternatives, as we will show, is favoured by a combination of system-internal reconstructive considerations, cross-dialectal comparisons, and evaluation of the sociolinguistic and geolinguistic dynamics of the area.