Oh my god!!!!!!!!! What are you talking about Vietnamese. You think that they love some gifts like that??? Like money$$, no, that is too wrong except on Luna New year for kid( they call it is " lucky money") like almost Asia country.
Ok, I agree with you that somethings in US like Nike .. is cheap for American, but do not thind that becasue VNese standard living are lower than you, that means is they can't buy it. No no no, they can if they want, i can make sure with you that now in Vietnam, people are very very smart...
Anyway, in my opinion i think you can bring what do you want, that will be great if you think its good, because people are very nice. If you have invited to have meal, no need to bring food more, you must come only, don't worry , no problem with thier life style.
So, the best thing I think is some thing that only US has ( not like Nike...) , ex: picture,card, insignia ...( somethings about your country, your life style..)