The process of second language vocabulary acquisition (L2VA) is investigated by looking at the learning, teaching, learner and environmental factors that affect the ways that adult Vietnamese learners of English acquire, store and use words. Data were collected by examination of informants' diaries, recorded classes, free conversations, interview-questionnaires and regular interviews during the English program. The data show that the process of building up the mental lexicon is slow, long and complicated. For adult learners coming from different English and job backgrounds, full time classroom learning is the biggest and most important source of L2 word input. It is also in this environment that the word storage and recall mechanisms are most facilitated. The L2VA process varies according to individual learners at different levels, with different learning goals, motivations, determination, areas of interest and word learning methods. It is hoped that the findings of the study help increase Vietnamese teachers' awareness of how to teach English vocabulary effectively and how to help learners work out individually suitable word learning methods.