This study examines the relationship between organizational socialization tactics (context, content, and social tactics), employees' work engagement and their proactive behaviors (relationship building, positive framing and self-management). Based on a sample of 675 Vietnamese young graduated employees with highly academic background, under 35 years old and with less than 5-year working experience at their organization, the results of questionnaire survey showed that institutionalized socialization tactics can increase Vietnamese employees' proactive behaviors and their engagement to work, and in its turn, work engagement encourages proactive behaviors at workplace. Among three types of socialization tactics, social tactics were the strongest predictor on all proactive behaviors. Our results also documented a partially mediating role of work engagement in the relationship between social tactics and proactive behavior, and fully mediating role in the relationship between context, content tactics, and proactive behavior. Our results hence support the interactionist perspective in that what happens in work settings is not a result of sole organization's practice nor individual effort; rather, it is the outcome of the interaction between the organization and the individual.