I am travelling from Australia to China to Vietnam to Cambodia to Australia. I have two passports, namely Russian and Australian (the first and last names are identical in both but the numbers are obviously different). I purchased all required air fares using my Australian passport. Later I found that as a Russian citizen I can enter and stay in Vietnam without a visa for 15 days (and as a holder of Australian passport I need to apply for a visa, which is extra $$$).
I can't see my passport number on the air ticket print-out, although they did asked for it at the time of purchase, so it might be in the databases. Does it matter that I used my Australian passport to purchase the airfare if I want to travel from China to Vietnam using my Russian passport so I can enter the country without a visa? Assuming airlines check for both passport number and the name details, should I present both passports when checking-in in China? I still have time to apply for Vietnam visa using my Australian passport, but it'd be great to learn I do not nee to.