Do I have to specify in my application which airport I am to pick up the visa

Hi, I understand there are 3 international airports for visa on arrival. Do I have to specify in my application which airport I am to pick up the visa ? Is it possible for me to request the Visa be issued in either the Hanoi or HO Chi Ming airport? If the visa letter specifies e.g. airport in Hanoi, and there is a last minute unexpected change in the airport of arrival e.g. a switch from Hanoi to Ho Chi Ming, can I still get a visa on landing with the visa letter for Hanoi? ( I am with a Hong Kong tour group that is to land in Hanoi airport but there is a possibility that the company may decide to reverse the itinerary and land in Ho Chi Ming first ) I look forward to your reply before filling out the application. Thank you. Diana