what is the maximum length China multiple entry tourist visa that a USA citizen can get in Saigon?

Usually, a multiple entry visa is valid for 6, 12 or 24 months and each entry usually allows one to stay in China for 2 months. So if you hold a multiple entry visa valid for 6 months, you may stay 8 months in china at most and 14 months at most if it is valid for 12 months and 26 months at most if it is valid for 24 months. FYI: https://www.travelchinaguide.com/embassy/visa/entry.htm
Some US passport holders get multiple entry tourist visa with duration of stay for up to 3 months.
BTW, if you don't have vietnam residence permit, it's almost impossible for you to get a multiple-entry China visa there.