Port of exit

Hi
 I'm up to travel to vietnam. I already got my visa and all the  documentation required. My question is: is that true that I must leave  the country from the same airport where I entered? If not, what should  write in my form when they ask me "port of arrival abd departure"? Like  (example) arrival: hanoi, departure: ho chi minh?

 Thank you for your time

 Regards

 Angela Gregori