No one wants to waste a day or two getting sick on vacation so I understand your hesitation. My boyfriend and I each got sick once however we were there for 30 days. If it helps his issue probably came from this restaurant in Hanoi and for me I probably got it from a street vendor in Hoi An so who's to say what's better. Our doctor gave us Dukoral that we took a week or so before we left on our trip which probably helped and we had 'emergency pills' (I don't remember their name) that we brought with us in case our illness got really bad (it didn't).

The best thing I can say from personal experience is try a vendor that offers hot food like pho or find street vendors in busier areas where the food might not be sitting out too long. I've heard also to stay away from vegetable toppings as they may not be washed in the cleanest water. Hope that helps!