The fruit in Vietnam is unbelievable. Fresher and sweeter than anything you can find in the US, it would be a shame not to gorge yourself on fresh fruit, while there.
From watermelon (Allison must have eaten at least twice her weight in watermelon during our month-long trip) to longans, super-sweet bananas to the freshest, best durian you will ever eat (even if you think you hate durian, you MUST try it if you're ever in Vietnam), the fruit is unbelievable. We ate it for breakfast, we ate it for snacks, we ate it for dessert. We just could not get enough of it.
These cherimoyas are a good example of the amazing fruit there. Known as "ice cream fruit" for it's texture and incredible flavor, it's great both on its own and in smoothies.