One of the biggest complaints about living in Vietnam is the heat, this is from both Expats and Vietnamese. A cool day in Vietnam is roughly in the low 80s F (high 20s C) which is a hot summer day from where I am from in Seattle.
One good way to acclimate is to NOT use the air conditioner whenever possible. At home, I try not to use the air conditioner until I go to bed. I then leave it on and sometimes will set timer to turn the aircon off during the night.
If you live in a nice quiet alleyway with a good breeze, you can just leave your windows and have a nice fan blowing on you. I cannot do this now since my alleyway gets real loud at 6 AM in the morning.
You will notice that when you start to rely on the air conditioner less, the outside air temperature seems to cool down a bit. For me, I consider 78 F (24 C) as cold now.
Lastly, another great way to acclimate is to trick your mind. I always set the temperature on my mobile phone or web browser to Celsius. When I see 30 Celsius it seems like a cool temperature even though it is actually 88 Fahrenheit!!!