Is it possible to get a free 15 day visa upon arrival at Hanoi airport?

No, that isn't true.

Vietnam doesn't operate a true "Visa on Arrival" system. In additional, all visas for Vietnam have a fee. Some nationalities can enter without a visa, paying nothing and being allowed to remain for 15 days, but neither of your nationalities s are included in that list. Your options for entering Vietnam are:

• Apply for a visa in advance from the relevant Vietnamese embassy. This page from the Vietnamese government may be helpful.
• Apply for a "pre-approved Visa on Arrival", where you initally pay a fee to a third party company (who provide an approval letter), then pay at a desk at Ho Chi Minh, Hanoi or Da Nang airport (US dollars only) to receive your actual visa. This service is not available at other points of arrival in to Vietnam. You can find out more on this Tripadvisor thread. Note that while there are many legitimate agencies who can provide this service, there are also a lot of scammers.

You will not be able to board a flight to Vietnam without either a valid Visa, or a stamped and approved invitation letter for the day you are arriving.