Many studies have demonstrated that vocabulary size plays a key role in learning English as a foreign language (EFL). In recent years, mobile game-based learning (MGBL) has been considered a promising scheme for successful acquisition and retention of knowledge. Thus, this study applies a mixed methodology that combines quantitative and qualitative approaches to assess the effects of PHONE Words, a novel mobile English vocabulary learning app (application) designed with game-related functions (MEVLA-GF) and without game-related functions (MEVLA-NGF), on learners' perceptions and learning performance. During a four-week experiment, 20 sophomore students were randomly assigned to the experimental group with MEVLA-GF support or the control group with MEVLA-NGF support for English vocabulary learning. Analytical results show that performance in vocabulary acquisition and retention by the experimental group was significantly higher than that of the control group. Moreover, questionnaire results confirm that MEVLA-GF is more effective and satisfying for English vocabulary learning than MEVLA-NGF. Spearman rank correlation results show that involvement and dependence on gamified functions were positively correlated with vocabulary learning performance.