For many teachers and administrators, the degree to which attrition over summer vacation represents a threat to instructed language acquisition remains unclear. In a previous study, Kramer et al. (2019) looked at receptive vocabulary knowledge attrition over summer vacation, found no evidence of attrition using these measures, and called for future research to instead use tests of productive vocabulary knowledge which is more likely to be forgotten. Therefore, in this study we investigate the amount of summer attrition among Japanese university students (N = 81) and any mediation in that attrition attributable to digital paired-associate vocabulary studying, extensive reading, or experience travelling abroad. The results indicate that although there was no significant group difference in pre- and post-test productive vocabulary scores, a small but significant relationship was found between digital paired-associate vocabulary studying and vocabulary test score gains.