I need more informations to give you some suggestions because in Vietnamese, traditional music is a huge genre includes pretty much songs.
If your parents like songs sung in Vietnam war (to stimulate soldiers), you can find these songs on Youtube: "chiếc gáºy Trưá»ng SÆ¡n", "Bác Äang cùng chúng cháu hà nh quân", "cô gái Sà i Gòn Äi tải Äạn", "Trưá»ng SÆ¡n Äông, Trưá»ng SÆ¡n tây", "cô gái má» ÄÆ°á»ng", "lá Äá»", etc. (we call this kind is "red music").
If they like soft and romantics songs (yellow music), then you try "thu quyến rÅ©", "mưa rừng", "thà nh phá» buá»n", "tình lẻ loi", "gá»i gió cho mây ngà n bay", "dư âm" (this is my favorite song, pretty good), "Äêm nguyá»n cầu", "ai ÄÆ°a em vá»", etc.
Other songs that are famous in Vietnam composed by Trá»nh Công SÆ¡n (have you ever heard his name?): "hạ trắng", "má»t cõi Äi vá»", "má»i ngà y tôi chá»n má»t niá»m vui", "diá»
m xưa", "em còn nhá» hay em Äã quên", "gá»i tên bá»n mùa", etc.
The songs above are hard to sing (I think) but let watch an English man sings a Vietnamese song, then you will know how to sing them: http://www.youtube.com/watch?v=0LQFVPMA9rE