This study investigates factors affecting teachers' intention to use the Zalo app--a social media with impressive users in Vietnam in recent years. The extended technology acceptance model (TAM) involves subjective norms (SNs) (colleagues, managers, students, and parents) and anxiety as the precursors of user attitude and intention to use as well as perceived ease of use (PEOU) and perceived usefulness (PU) as the key variables in TAM was employed. 1,105 teachers in Vietnam took part in the online survey. The study employed the partial least squares structural equation modeling (PLS-SEM) to analyze the quantitative data and the relationship among factors. The findings show that colleagues have no impact on PU, and managers have an insignificant influence on PEOU. In contrast, students and their parents positively influence teachers' PEOU and usefulness. Moreover, managers can increase teachers' anxiety levels, whereas students' connection decreases anxiety. These variables accounted for 79.6% of the variance in users' adoption. The results confirm the impact of SNs on teachers' intention to use the Zalo app in working. This is the study on issues in Vietnam related to social media used at institutional level--a no way back solution in the new educational context of the modern society.