In this study, we analyze tautosyllabic vowel nasalization in Santomean Vernacular Portuguese (SVP) spoken on São Tomé Island. In European and Brazilian varieties of Portuguese, vowel nasality has been considered both a phonological (actual nasal vowels) and a phonetic (nasalized vowels) feature. Herein, we share an acoustic analysis demonstrating that nasalized vowels in SVP are 48% longer than their oral counterparts. Thus, a nasalized vowel could correspond to an underlying sequence of an oral vowel followed by a nasal consonant. In this sense, a nasal coda's nasal feature would spread to the continuous vowel on the left. After that, the nasal coda, which triggers the process, is deleted at the segmental tier without changing the syllable length. This suggests a biphonemic interpretation of vowel nasality in SVP. Accordingly, there are no phonological nasal vowels in SVP. As São Tomé and Príncipe is a multilingual country, we investigate the influences triggered or reinforced by linguistic contact between Portuguese and other languages spoken in the country, especially Santome.