The present study investigates the impact of bidialectalism on L2 production, focusing on the role of dialect modes. Shanghai-Mandarin Chinese bidialectal speakers were recruited to produce second language (L2) English vowels under the influence of either Shanghai or Mandarin Chinese mode. Results showed that in the Shanghai mode, participants' English vowel production closely resembled that of native English speakers. Notably, Shanghai Chinese significantly influenced English vowel production in the Shanghai mode, and Mandarin Chinese had a strong impact on English vowel production in the Mandarin mode. This highlights that each first language (L1) dialect--that is, the activated dialect mode--significantly influences L2 English vowel production. The present study reveals that bidialectal speakers have differential L2 production performance depending on the L1 dialect mode that they activate. These results are interpreted within the framework of the second language linguistic perception (L2LP) model, contributing to the theoretical understanding of how L1 dialect modes influence L2 acquisition.