Determine what are the subject you want to shoot in vietnam. Landscape? bring 17-40. Street shooting ? bring 28 - 135. In my opinion, 70 - 200 may not be necessary for travel use. The best is to bring both 17-40 & 28-135. But if you want to bring only one lens, choose 28-135. For me, I would bring 17-40 & couple with a 50mm.