The proper way to offer your card is to hold it by the corners with both hands. While 35mm print film is widely available, it may not always be fresh or properly stored. Slide film may be difficult or impossible to find. Money belt or fanny pack. As you will soon read, cash and small personal items are attractive to pickpockets.
Leg wallets are uncomfortable.
The Socialist Republic of Vietnam is understandably very particular about the things which it will allow visitors to bring into the country. Explosives, firearms, seditious propaganda, or items that are culturally offensive are positively forbidden. Culturally offensive material includes books and magazines which customs officials may determine to be dangerous, including books or magazines with nudity. You may not take video tapes, audio tapes into Vietnam without the permission of the customs inspector. Illegal drugs of any kind are absolutely forbidden and possessing them could cost you your life.