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