While vocabulary learning strategies (VLSs) have primarily been taught in physical classroom settings, the potential for applying strategy training in an online environment remains under-explored. With the rising acceptance of remote education, there is a need to investigate the impacts of applying strategy training in online contexts. Hence, this study explores the effects of online deliveries on VLSs and vocabulary usage in writing among adult English as second language (ESL) learners via massive open online course (MOOC) and video conferencing platforms. A total of 50 pre-intermediate ESL learners completed a 5-week VLSs course on a MOOC while 44 students learnt from the instructor synchronously using Microsoft Teams. The data comprised questionnaires and learners' compositions, which were collected before and after treatment. The data analysis procedures involved Lextutor's version of lexical frequency profiling (LFP), descriptive and inferential statistics. The results revealed that there was a significant increase of usage in all categories of the VLSs after the course for both groups. Additionally, the effects of VLSs instruction on vocabulary usage could be reflected in both groups' writing though the choice of words might have been task-influenced. The findings from this study could yield useful insights on online vocabulary learning for ESL instructors and learners.