Vietnamese pronouns may be the most challenging part of the language for foreign speakers. Even for native Vietnamese, finding an appropriate form of address could be tricky in many situations.  The reason is Vietnamese pronouns vary depending on age, sex, social position, level of intimacy, and close or distance relationships betwen the two people in conversation. In other words, forms of address are always specified by the context of a conversation.
By providing a bunch of examples with detailed explanations I will try help you to understand the principle. Then I'll summarize all Vietnamese personal pronouns used in different context. So you can start your adventure in  substituting  English personal pronouns with appropriate Vietnamese ones. A table of the most commonly used personal pronouns is available at the end of this section.
 
Ex 01:
Two strangers' talk, please note the use of  o^ng , to^i
A: Ho^m nay tro+`i dde.p qua' ha? o^ng ?
    What a nice day, isn't it ? (lit: What a nice day,  do you agree ?)
B: To^i nghi~ la` tro+`i sa('p sang thu.
     I think the fall's coming.
In this conversation the pronouns o^ng (you formally) and to^i (I, me formally) were used due to the fact there is no relationship whatsoever between these two men. However by using the formal form of address we can depict this conversation took place between two older men of comparable age.
 
Ex 02:
A talk between a young couple, please note the use of anh, em
Young man:      Em thi'ch ddi coi phim to^'i nay kho^ng ?
                        Do you want to go to see a movie tonight?
Young woman: Anh thi'ch coi phim ha? Em thi'ch ddi nghe ho`a nha.c ho+n.
                        You like to see a movie ?  I prefer to go to a concert
A woman addresses herself em when talking with her husband or lover in most of cases. She calls him anh. A man uses anh to address himself and calls her em.  This form of address is also used in family between older brother and younger siblings. It is widely used in informal conversations between young, middle age men and young, middle age women.
 
Ex 03:
A conversation between father an son:
Father:    Con dda~ ho.c ba`i xong chu+a   ?
               Have you done your homeworks ?
Son:        Da., con ho.c xong ro^`i, ba cho con xem dda' banh nha !
               Yes dad, I am done, may I watch soccer now ?
A child must be con in relation with parents without exception. However parents are addressed differently from region to region. In Southern Vietnam  father is calles ba, in Northern Vietnam bo^' . Mother is called ma'  in the South and me.  in the North. Ba can be used in pair with either ma'  or me.  to refer  to both parents. Bo^'  goes exclusively with me. only . In literature, the formal way to call father is cha , mother is me. and cha me. to refer to both parents.
Note: Vietnamese speakers always use familiar titles as personal pronouns while talking with family members. In the above example con litterally means child, ba means father. That's why a child talks with parents, he or she always be con.
A foreign speaker can stick with the simplest form of address provided at the end of this section in most conversations.
 
Ex 04:
A talk between two close friends:
A:    E^ ddi dda^u ddo' ma`y ?
        Hey where are you going ?
B:    Tao ddi ddo'n con. Ba` xa~  ma`y kho?e kho^ng ?
        Pick up my kid. How's your wife doing ?
Tao is used as the first personal pronoun in very informal conversations between close friends, colleagues, school mates ... in short between those of comparable age and background. Ma`y is the second person. This pair always goes together. It makes no sense in a conversation if  you address youself tao and your partner o^ng or anh .
Ex 05:
 
A business conversation
A:    Chu'ng to^i muo^'n ky' ke^'t ho+.p ddo^`ng na`y vo+'i ca'c o^ng, nhu+ng gia' cu?a co^ng ty ABC co' re? ho+n.
       We want to make this contract with you, but ABC Inc's price is lower.
B:    Ca'c o^ng ddu+`ng lo . Chu'ng to^i se~ co' gia' tha^'p ho+n cu?a ho. .
       Don't be worried. We can beat their price.
Chu'ng to^i  is used as we in English in most situations. It is created by adding the word chu'ng (literally means more than one)  to the word to^i (I). Similarly Vietnamese have the word chu'ng ta , which also means we,  but by saying this, a speaker refers implicitly to all participants in the conversation. Ca'c o^ng is plural personal pronoun you. It is created by adding the word ca'c (literally also means more than one) to the word o^ng. Because o^ng is the formal form of address of you, ca'c o^ng  is the formal form of address of you plural.