Do we need transit visa for Guangzhous airport on the travel to Vietnam?

Dear Embassy of the People's Republic of China in Japan
We are a family from Sweden, now living i Tokyo for three years, are "japanese residents". We plan to travel to Vietnam this vinter and go with China southern airlines, via Guangzhou airport. We change fligth on both ways but on the way back to Tokyo we arrive in evenings to Guangzhous airport and left it next morning. Do we need to have transitvisum? Are we able to leave the aiport the evening on the way back to Tokyo? Anything eles to think about. I suppose it´s ok to fix with the transitvisa if we need it, after we have bought the flight.
Best regards
Eva-Lena Bustrén