Will my outbound flight from Da Nang instead of Hanoi affect my visa application in Vietnam?

No. But if they ask you to come you need to come.

Proof you're in Hanoi region upon application.

It's good if you show proof of accomodation booking and payment, but maybe not 'required' in Location proof. It's one of the options of proof. I did give give proof of all in one go cause why not? Have the proof cause it's real: flight, accomodation, Timestamp photo in front of Hanoi site.

But, yeah, do as you like.

Cause no need to proof return journey, only intended arrival date/flight to Thailand.

But...

You want DTV or DaNang ?

Maybe you can have both as you want.

But...

You want to lie about intended departure place?

Or be honest and kinda admit you're just using Hanoi cause it's faster but actually gonna be staying in DaNang, unavailable for possible face-to-face (which seems a thing from the past anyways) cause DaNang is your comfort zone?

IDK

Whatever

None of the officer from embassy is tracking the applicant in every minutes.

Also there’s no rule to be in one place.

But the applicant must be inside Vietnam country during the visa processing. Not leaving the country until approved or rejected.

Expect one possible request from embassy; is to take picture of where you are located. They might check where the applicant is. That is real request and also depends.

One of my friend’s friend applied in HanNoi and a few day later came to HCM to visit. Unfortunately he got rejected. I don’t think because of that likely to be coz of documentation.

I think you must Stay in Vietnam until your visa is approved