Hi Ken
Temperatures on the the National Parks holiday will be quite varied. It could be as hot as high 20s during the day but drop to a cool 10 degrees during the night.
Vietnam will be hot and humid - most likely 25-30 degrees with quite a few showers.
So if you like hot weather, I would go for Vietnam; if you don't, go for the National Parks.
Kind regards,
Olly, your editor