Neural Machine Translation (NMT) has gained increasing popularity among EFL learners as a CALL tool to improve vocabulary, and many learners have reported its helpfulness for vocabulary learning. However, while there has been some evidence suggesting NMT's facilitative role in improving learners' writing on the lexical level, no study has examined whether vocabulary improvement made with the aid of NMT leads to any vocabulary retention when the tool is no longer in use. The present study employed a quasi-experimental design to examine the extent to which editing with NMT may lead to vocabulary improvement, immediate vocabulary retention and delayed vocabulary retention in the legal context and how the findings may vary by learners' proficiency level. ANOVA results revealed that learners with higher proficiency achieved a similar level of immediate and delayed retention compared to their vocabulary improvement, while the benefits of NMT for learners with lower proficiency were significantly confined to vocabulary improvement and immediate vocabulary retention.