Are you sure it was a fair election back then while millions of South Vietnam soldiers were being imprisonment in Concentration Camps? However, it is besides the point here and to reply to your questions:
If people of New VN did not chose Pham Van Dong as the PM, then Vietnam government still would be responsible for its former PM's signature. Why? Pham Van Dong had signed the letter on the behalf of his government at that time, had he not!?
1. MP Pham signed somthing
did not belong to North VietNambefore 1976, so Ok VN government will say sorry for that mistake,and hope China should not ask new VietNam to accept any thing esle doen't belong to VN any more.
2. If China claim US belong to China and wanted Pm Pham accept it. PM Pham worte:" VN eccept that US belong to CHina coz CHina discoverd American continent earlier than the West", so pls tell me what responsibility should VietNam take for his mistake sir ??