Shanghai (PVG) Transit. Would these trips qualify for transit without visa?

Hello,

I am a US passport holder that arriving in Shanghai (PVG) from Los Angeles . I will stay in Shanghai 2 days and fly to Hong Kong. I will then leave from Hong Kong to visit Vietnam. On my return home from Vietnam to the United States, I will fly from Ho Chi Minh City to Shanghai. I'll have an 8 hour layover in Shanghai before flying back to Los Angeles.