Should change Yen in your country first is better (because exchange rates are better in Japan). If traveling in Japan, you should not change Vietnamese currency to UD dollar money, and then have to change from USD to Japanese Yen, you will lose the conversion fee 2 times. Refer to Currency in Japan to plan your spending by cash or credit card.
Changing money in Japan is not easy to change money, you must go to banks or legal exchanges. Banks are open from 9am to 3pm. Saturday, Sunday and major holidays the bank thinks. You can also exchange money at major food stores, but the price will be higher than the transaction points.
Mobile phone and Internet
Mobile phones are almost inseparably awake in modern life. However, in Japan, you should not think about buying a prepaid SIM card at the airport like in some countries. In Japan, most people use postpaid SIMs, they must register with the network operator. Also should not use roaming service (ROAMING), because the cost is very high (this is advice only, of course for travelers who do not care about the cost, feel free to use).
The best way is to connect to Wifi and use Internet calling services like Viber, Zalo, Whatapps. In Japan there are free Wifi available now, but the registration is tiring, and the free is of poor quality, so the best way is to hire a Wifi hotspot. If you can only be reached by traditional phone lines, please refer to renting a phone in Japan.
Shopping and note about electronics
Types of electronics, computers, digital cameras, if purchased in Japan, may be cheaper than other countries, besides the quality is also better and more assured. You can rest assured, any of its money and not afraid of counterfeit goods. However, please note that the instruction manual may not have English.