In both countries, it is recommended (and sometimes obligatory) to cover your knees and shoulders when entering Temples and Pagodas, so if you are planning on wearing shorts, carry a pareo with you
Food is delicious, do not miss out on local recipes
When in Hoi An, search for beautiful pieces of silk (either clothing or lamps)
If you are interested in jewels, this is the right place for you
Try to always have water with you
Try at least once a trypical Banh Mi, the local sandwich
The Yellow Backpack 