Can you enter China on a single entry visa and then enter again to transit w/o visa?

I plan to visit China in November for 4 days and then travel onto Southeast Asia. On my return journey to The US, I plan to fly from Hanoi to Los Angeles via Shanghai. My understanding is that I need a tourist visa for my first entry into China but I'm unsure as to whether I can transit without visa in Shanghai on my return journey or do I need a multiple entry visa?