Giving face is a very important concept in Vietnam. You must give the appropriate respect according to rank and seniority. For example, if you are buying gifts for an initial contact, make sure you buy better gifts for the senior managers instead of buying similar gifts across the board.
Similarly, sitting positions in a meeting room or a dining table is accorded accordingly to rank, importance and seniority. It is good to seek advice before embarking on your first meeting with Vietnamese business contacts to avoid making the wrong move.