Can US and Fijian passport holders apply for Visa to travel to China in Hanoi Embassy? How long?

Four of us are travelling to Hanoi on American, New Zealand and Fijian passports in March for ABB Hanoi Transformers testing. We need to apply from there for ABB Xiamen switchgear testing. We are purchasing these equipment for installation in American Samoa.

Can all the above passport holders apply for Visa to travel to China in Hanoi Embassy. How long will it take?