What is the coolest temp in Vietnam?  

It depends on the region. Up north, it gets the coolest with temps averaging around 9 °C (48 °F). It can be colder but it is rare to see it go down to freezing unless you are up in the mountains themselves. 

I didn't know it went below 13°C! I guess it does make sense if you are more up north because the mountains tend to pull in cooler air. The more you know! I learn something new every week about Vietnam.  

The coolest I have ever experienced when I was there was 58 degrees. It felt amazing after having gone when it was tipping triple digits so now I only go when it gets cooler outside. at least at night.

The average cool temps tend to be around 58 in most areas but it can get below 50. I have never been in Vietnam where it had felt very cold at all. 

It will always be cooler up north vs the south but most of the area stays fairly warm compared to other countries.