The phrase for Communism in Vietnamese is "Co^.ng Sa?n", meaning "combining properties". Supposedly, Communists should live in communes and share properties. Ideally, everyone is equal, no rich, no poor. The Vietnamese Communist government tried to achieve that goal by having a few changes of currency. While I lived in Vietnam, I was aware of only one change of currency (ddo^?i tie^\n), but other Vietnamese have told me that more changes happened after 1979.
On the day of the change of currency, the government issued a new type of paper money. Everyone was supposed to bring in all their monetary possession to some central place in their neighborhood and regardless of how much each bring, the maximum any person would get was 200 units of the new currency. Ideally, after the change everyone would be equal, albeit equally poor. I am sure the government would print any amount of money as they need it themselves.
All of a sudden, much of the old paper money became useless. The coin money was still in use though. We happened to have some uncooked sweet rice (ga.o ne^'p) at home, so my mother made sweet rice and sold them to neighbors and others, in exchange for coins. People who normally threw away spare change then had to scourge around the house looking for the precious coins.
Our next door neighbor, Mr. and Mrs. Kam, were relatively richer than us and had more than the allowed amount, so we helped them converted some of their money to the new currency. Let's say the maximum amount anyone could bring was 1,000, and the Kams had 1,500 while we only had 500. The Kams "gave" us the extra 500 so we would meet the maximum too. I think Mrs. Kam offer to give us some money for the trouble but my mother turned down the offer. The Kams were among those who normally don't bother keeping their spare changes, so for a few days they were penniless. My mother gave them some of the sweet rice.