Research has established positive associations between informal digital activities and vocabulary knowledge. However, understanding how different types of digital activities relate to various aspects of vocabulary knowledge is limited. This study examined how the purpose of informal digital activities and strategic engagement during and/or after technological activities relate to receptive (meaning recognition) and productive (form recall) knowledge of form-meaning links as well as knowledge of word association. Structural Equation Modelling analysis of the questionnaire and vocabulary test data from 339 university EFL learners revealed that two purpose factors (informal digital activities for instructional and socialization purposes) and two strategic engagement factors (dual focus on meaning and form; depth of lexical processing) were key determinants of form-meaning mapping. The interplay of purpose and strategic engagement manifested differently for receptive and productive knowledge of form-meaning links. In contrast, depth of lexical processing was the only predictor of knowledge of word association. The findings underscore activity purpose and strategic engagement as crucial factors in researching and bolstering the relationship between informal digital activities and different aspects of vocabulary knowledge.