If you want to enter Vietnam, we highly recommend you to apply for Vietnam visa first and buy your airline ticket later since it will be safer.
However, when you apply for visa for certain countries, the embassy of these countries in your country may require you to make a flight reservation. In this case, you just need to call the booking-office and book your flight to get visa. You do not need to pay the ticket but if you are sure that you can get visa and you have an urgent task, it is impossible to receive immediately the ticket.
In contrast, if you cannot be 100% sure about getting visa, paying airline ticket can cause many damages. After the confirmation of the embassy about your visa, you should ask for your ticket in order to get visa because if you do not have airline ticket or your airline ticket is different from ticket booked, you will not get visa at the embassy.
What will I do if I lose my airline ticket?
If you lose your ticket, you need to contact immediately airline and fill out a lost ticket application by giving your personal information (full name, flight date, booking day, telephone number). The airline will check your information and issue a replacement ticket. You usually have to pay some money for service charge or penalty.
Vietnam visa: Buy airline ticket before getting visa, should or should not?