The rapid development of high technology and the Internet has made online learning increasingly popular, especially in the last few years when the COVID-19 pandemic appeared and spread worldwide. However, implementing online English courses in the pre-COVID-19 period was not prevalent; and only a few courses utilized new technologies to teach the English language online in universities and colleges in Viet Nam (Pham, 2020). This paper investigates 270 Vietnamese tertiary students' digital competency and online English writing study and practice experience in pre-, during, and post-COVID-19 periods. It also examines the link between the lexical features and the quality of the students' online English writing. In addition, the study surveys English teachers' perceptions of lexical features' influence on the students' online English writing quality. The study found that Vietnamese tertiary students' digital competencies are of a high standard, with plentiful experiences of using digital technology in learning and practicing English. Some students faced challenges such as performing digital devices, communicating with friends and teachers, reading guides and documents in English, or having financial issues while taking online English courses during the COVID-19 pandemic. However, most of them solved all problems quickly and effectively themselves. Moreover, the findings implicated the strong influences of lexical sophistication, lexical diversity, and language accuracy on the students' English writing quality. Finally, it showed teachers' approval for students to use English writing aids when learning or practicing writing English online because of the educational benefits and convenience they could get in online English learning sessions.