US - Shanghai - Vietnam - Return to US with connection in Shanghai?

Hello, I have a US passport. I am planning to travel from San Francisco to Shanghai and stay for 3 days. Then from Shanghai, I plan to fly to Ho Chi Minh City. I will only have a VOA letter for the Vietnam Visa requirement. Is that ok? Or should I try to get the actual visa from the Vietnamese Consulate?
Second question is can I have a layover in Shanghai from Ho Chi Minh City for my connection back to San Francisco?
All of my flights will be on the same itinerary on China Eastern Airlines.
Thank you for your response!