Vocabulary mastery is critical to English as Foreign Language students. Mobile technologies enable students to learn vocabulary without space and time limitations. However, existing mobile-assisted vocabulary learning research often employed teacher-directed activities that increased instructors' workload, undermined student motivation or targeted individual cognitive outcomes only. In this study, a Contribution-oriented Self-Directed Mobile Learning Ecology (CSDMLE) model is proposed for developing student-directed and motivational vocabulary learning activities in groups. Through a mixed-method design, we administered a survey and a vocabulary test to 55 freshmen students in a Chinese university, and conducted follow-up interviews. We found that students in the CSDMLE group outperformed those not in the group in the post-test vocabulary test, but there was no statistically significant difference between the two groups' delayed vocabulary test or L2 motivation. However, the treatment group displayed a highly favorable attitude toward the learning approach and a strong intention to use it continuously. The findings have implications for technology-supported vocabulary learning activities.