Do i need transit visa? Or just apply for 1 day permit

I am Vietnamese citizen living in Japan. In August I will fly to Ho Chi Minh from Osaka, having 20 hour transit in Shanghai. I would like to go out and stay 1 night in the city. Do i have to apply for transit G in advance or just apply for 1day permit stamp at the Immigration in the airport as shown in Chinese Embassy website.
I read many different informations about this so i am really confused.
I want to avoid some troubles on my journey. So please help me.
Regard