I have a flight from New Delhi to Ho Chi Minh City in Vietnam, and From Ho Chi Minh I have a connecting flight to Hanoi but with different airlines (domestic). Which should I write as "entry port" in my evisa application? Ho Chi Minh or Hanoi?
Your port of entry is Ho Chi Minh City. You have written yourself that you will take a domestic flight with a different airline later. That is just internal travel in Vietnam. You will enter Vietnam at Ho Chi Minh city. There have been social media posts where some people were denied boarding in India whose destination on the ticket was different to the port of entry mentioned on their e-visa.