Use honorifics and kinship terms rather than first names alone, especially with older people or anyone in a formal setting; common forms include
anh,
chị,
cô,
chú, and
bác, chosen by the other person’s age and relative status. In business or official contexts, address people by their title plus given name or by their full name, and wait for them to invite you to use a more casual form because direct first-name use can feel overly familiar.