" This question can be used to ask someone how he or she is after that person asks you first and you have given your response.   Et means "and." The question literally translates into, "And you?" You can use this question with anyone and in any situation, but it is especially used in formal situations or with strangers and elders. " This question is also used to ask someone how he or she is after that person has asked you first.   Toi is an informal way of saying “you.” This question is used in casual, informal settings. Only use it with friends and family. Use "et vous?" in a more formal setting.

TL;DR in Vietnamese:
Lịch sự hỏi lại rằng "Et vous? Hỏi bạn bè và người thân rằng "Et toi?