This study describes and analyzes the low vowel [a] heterosyllabic nasalization processes in two Portuguese varieties spoken in São Tomé and Príncipe (STP): the Santomean vernacular Portuguese and the Principense vernacular Portuguese. STP is a multilingual country where, in addition to Portuguese, creole languages are spoken, including Santome and Lung’Ie. Based on an experimental methodological approach, we performed acoustic and perceptual analyses that included measuring the duration of the segments under analysis and the formants that comprised them. In addition, this research contains a phonological study that considers primary stress and formation of foot in which heterosyllabic nasalization occurs. The results indicate that heterosyllabic nasalization is optional for the low vowel in a stressed position, marking a difference concerning European and Brazilian Portuguese, variants in which this process is mandatory in similar contexts. These results suggest an approximation to the native languages and characterize the studied varieties as distinct.