Should I have Visa or Letter before depart from other country to go in to Vietnam?

I'm Korean and now I'm in Laos. I had a Vietnam visa until 28th Aug, 2015(now expired already), and I departed from Hanoi on 17th, Aug, 2015.
Today, when I went to the airport in Laos, the Vietnam airlines staff said I can't take the plane, because I try to go back to Vietnam within 30 days without Visa.
So, I got helped from my Vietnamese friend and I can get my visa on 3rd Sept, 2015 before 2pm at the HCMC airport. (I will arrive at the HCMC at 3pm).
However, the staff in Vietnam airlines said that I should have the letter or the visa before check-in at Laos airport. If I don't have a letter or visa, they can't let me take the plane..... I can't understand.
So, I will get my Visa at HCMC airport and Vietnamese immigration will take care of it. My Vietnamese friend and I wonder why the airline staff in Laos asked me about Visa or the Letter and doesn't let me take the flight.....
now, I'm in a big trouble. I missed my flight today, and ruin my plan in Vietnam... please tell me that it is needed to show the visa or the letter to the airline. Thank you so much. please answer this asap.