You should plan a long time so that you can find out more and more interesting things.
FOOD
If you like junk food, Vietnam is a perfect destination. From North to South, you can find hundreds kinds of junk food from various material. All of them delicious, healthy and easy to eat.
Even the main food, you can find that almost is ok for you. Vietnamese food is not so oily, mainly vegetable, good tastes. The South food is sweet while the Middle food is salty.
TRAVELLING
No matter that what you like, you can find it all here: swimming at sea, climbing to mountain, sand hill, ...
As long as yu have enough time to go.