I think we have a little bit diference. In culture Americe, when someone begin a talk, he says "hey guys/ how are u ?" and the other one say " i'm fine, thanks" that mean he don't want to keep talking. And in Vietnam, if "Cảm ơn" before a sentence that's like same mean American culture.
For example , when u walk on the treet, a sales person suddently begin talking with u to sreve his work, u can say " cảm ơn, tôi không muốn" ( i don't want it, thanks)