When you study abroad, you will inevitably encounter physical challenges. While I was in Vietnam, the main challenges I faced were the heat, insects, and food poisoning. I believe that the key to overcoming these challenges is being prepared with helpful items and maintaining a positive attitude.
I highly recommend that you pack or purchase light, long pants (not shorts!), insect repellent, Pepto Bismol + Immodium, Benadryl anti-itch gel + Benadryl anti-itch stick, a fan, a face towel (for wiping sweat), a heating pad or hot water bottle (for food poisoning cramps), and a cooling towel. These items made my experience much more comfortable. However, getting mosquito bites, feeling really hot, and getting food poisoning will happen to you no matter how careful or prepared you are.
The key thing to remember is that all of those issues are temporary and that all of the other amazing experiences you will have in Vietnam will make everything worth it. Plus, your room and classrooms will have air conditioning (a true luxury!) and if you feel really sick your roommate or the CET staff can help you go to the local clinic. Finally, if the Benadryl anti-itch cream doesn't work on your bug bites, try using toothpaste. It works like a charm!