Funny you mention Vietnam, that was actually my favourite place. Loved the north, I motorcycled around from Hanoi to Sapa and even made it tot eh Chinese boarder. Just breat taking views, and so much different from the south. Ha long Bay was so much fun too.
Someone mentioned Costa Rica earlier, and since I brought it up for Y2K, I will for you as well. G is a great option, and can ease peoples fear for safety, and or where you are staying and normally a lot of you food would be included. Here is the link to Costa Rica, but they go lots of other places, and I can get you a discount on many of there prices.
Here
G adventures is just a great Canadian company, I know I have already talked about them a bunch, but Canadian company that is the only company to be backed by both national geographic and Jane goodal says something about the brand.
For Peru I wouldn't worry too much about the weather, it definitely won't be great, but that isn't probably why you are going there anyway. There is also a better site than Machu Pichu to go to, the name escapes me, it just isn't as famous.