You may not need a visa if you are a citizen of a country such as the US that has an exemption agreement with Japan:

http://www.mofa.go.jp/j_info/visit/v...rt/novisa.html

From what I understand there is also a "shore pass" available which you can use to transit between NRT and HND within three days depending on your nationality (Vietnamese nationals are covered). It is mentioned here:

http://visados.com/en/visa-for-Japan