Can I fly from HCMC, Vietnam to Shanghai and get a visa on arrival?

Actually, there is no visa on arrival in Shanghai, but you can create an itinerary that will be eligible for 144 hour visa free transit. The only condition is that immediate countries and eligible territories before and after your Chinese destination are in different countries or eligible territories. In other words, an itinerary in your case that would be eligible is (these are just two of several possible examples):

HCMC - Shanghai - Hong Kong - HCMC

HCMC - Bangkok - Shanghai - HCMC

If you create an itinerary of this kind, you will become eligible for 144 hour visa free transit and you will travel to Shanghai and back absolutely visa free.