Hello, I am Vietnamese. 
I also question myself whether or not Lung is a name??? 
It is quite awkward. 
For 30 years in my life, I have not met anyone who names Lung - Lùng - Lứng - Lửng. 
Though there still be two options. 
Firstly, it is a wild animal name Lửng (picture below), but nowadays we don’t name after an animal anymore. 
 
 
Secondly, probably Lung belongs to some ethnic minorities living far from modern cities.