Xiamen VOA for Japanese and Vietnamese citizens

You are definitely eligible for Xiamen VOA, but I am afraid that your girlfriend is not. Chinese VOA policy is somewhat specific toward ASEAN member countries. Some of them are eligible without any doubt like Singapore. Some are eligible by presenting their old, used visa like Indonesians and some, like Filipinos are not eligible. This is all valid at the time being. As far as the Vietnamese citizens are concerned, I would say that they would be eligible if they can present their old, used VOA. Since your girlfriend does not have one, I would suggest you not to try to board the plane because you could be denied boarding in Manila.

Also, you as Japanese citizen do not have to obtain any kind of VOA because you can rely on 144 hour visa free transit through Xiamen if your itinerary is:

Japan - Xiamen - Manila

You do not need to obtain any kind of visa since you are eligible for this visa free transit option that allows you to stay within Xiamen city area for up to 144 hours.