Background information
History

A universal wall socket that accepts plug types A, B, C, D, E, F, G, I and O.
Since Vietnam mixes two kinds of plugs and sockets, most outlets are hybrids that work with both. There are three types of universal sockets: the ungrounded version (which accepts types A and C), the grounded version (which supports types A, B and C) and a truly 'universal' version (see image) that accommodates no fewer than 9 plug types (types A, B, C, D, E, F, G, I and O). The last type, however, is not frequently installed.
Keep in mind, that despite using American-style plugs, Vietnam's electricity does not run at 120 volts, but at 230 volts. Because of this, tourists from North America and Japan — where the standard voltage is 120 volts — should make sure their appliances can handle 230 V. Always check your device’s voltage specification to avoid any issues: "INPUT: 100-240 V" means that your appliance is multi-voltage and that it will work with any voltage between 100 V and 240 V. Luckily, many travel gadgets (such as laptops or phone chargers) are multi or dual voltage, so most tourists will not have a problem. If your device is single voltage (e.g. "INPUT: 120V"), you will need a transformer to step down the voltage.
Click here for a global map showing the spread of the different plug types used around the world.
Click here for a detailed list of the countries of the world with their respective plug and outlet types, voltage and frequency.