The inability to differentiate the English vowels /o/ and / O/ has become a stigmatized marker of a lower prestige and widespread dialect of Sri Lankan English. This lower prestige (LP) dialect is often referred to with the derogative phrase “Not pot English”. This study aims to investigate the production of the vowel contrast by native Sinhala speakers of English. To this end, speech samples of three adult learners were analyzed. The findings of the study are discussed according to hypotheses of the Speech Learning Model, which suggests that the existent L1 specific phonetic categories hinder the formation of new L2 sound categories. Here, sounds that are similar, but not identical to L1 sounds are considered to be the most difficult to acquire. Also, the percentage of L1 use and the age of second language acquisition seem to have influenced the production of the vowels. Finally, in order to address this pronunciation issue, an instructional framework to teach pronunciation is proposed.