Hello, we already have our flights booked, from Germany to HMC and directly proceed to Cambodia. We want to stay for a few days in Cambodia and take a boat down the Mekong to enter Vietnam in Chau Doc by “land” and stay about 3 weeks in Vietnam before leaving back home to Germany from Hanoi. Can we apply for a visa on arrival, get an invitation letter and get the visum into our passport at the HMC airport, but not use it until we enter by land from Cambodia in Chau Doc? If no, do we need an invitation letter to get on the airplane in Germany, even if we proceed and transfer directly to Cambodia? If we do not need an invitation letter, we could get a visa at Phnom Phen which is much cheaper and more convenient? Thank you very much! Cheers, pascal