As of 2003, a great number of restaurants throughout Vietnam tend to use a lot of monosodium glutamate (MSG) in their food preparations. And the Northerners use a larger quantity than the Southeners. Even if you request no MSG, the MSG still may be added when the dish is cooked. One way to avoid much of the MSG is to eat the food in the dish or soup, but not  the liquid.

Most restaurants use cattle bones to brew their soup stock for use as a base in Pho (beef noodle soup) or Bun bo Hue (spicy beef noodle soup Hue style); but it's also highly likely that the soup stock is made with pork or chicken. You can judge what bones they used for the broth by the name of the dish . exp: Ga = Chicken dish with stock based on chicken bone. If you are restricted in the kinds of meat you eat, it's best to clarify.

When it comes to available fresh fruit, Vietnam is a tropical paradise: rambutan, lady finger bananas, dragon fruit, etc.  By western standards, the availability is also an amazing bargain. But be careful: the scale could have been recalibrated/tampered with so that it reads less. This can be a real problem when you haggle; if the price was low and the seller accepts, you may fall into this trap.