How can I copyright my story when I haven't written it yet?

You have a copyright to your outline, as written.

No one gets to own story ideas, though.

What you do, is don’t share story outlines with people you think are going to use your ideas in ways you don’t want them to. But it’s pretty rare that there would really be a story-writer who wants to use someone else’s story outline to write a story using it. The chances of someone doing that, and being in competition with you in any way, seems vanishing small, unless you have specific rivals who can write stories but can’t invent good outlines, and who are going to try to publish where you are trying to publish. And even if you did have some rival like that, they’d be making themselves ridiculously vulnerable to you outing them for stealing your story line (and for being crazy maniac story-outline bandits, which is ridiculous).

I wouldn’t worry about it. The odds of someone reading your outline and taking the time to write your story is infinitesimally small. The best protection is to write your story.

Lots of stories have been done with war theme involving every skirmish in recorded history.
What sets them apart is the creativity in the author’s presentation.
Lots of people know I have written an alternative view of the Cinderella story. That is no threat to my work. There are many interpretations of the story. What I don’t discuss is the specifics of what makes my story unique.

What you have voiced, but not written you cannot claim as yours, unless it has been recorded for the public.

My advice is to get it written, get it to market.
One thing you can count on is anything you have confided to me is safe. I am not one for writing battle pieces. It is not my genre. I am more fantasy, and Anthropologic lifestyles. It is a genre I prefer to read someone else’s perspective than try to develop my own.

So, apply yourself, get it done and marketed so nobody can beat you to it. I love the research you have done. You need to trust your talent. Don’t worry about what you have said. Just avoid saying any more.
You deserve to see this through.

Whatever you’ve written, even in draft form, is protected by international copyright agreements that Vietnam has endorsed. Look here for some useful info as you consider your publishing options:
https://kenfoxlaw.com/copyright-registration-in-vietnam/

The good news is that unlike many other developing countries, Vietnam has accepted rights & protections for a wide range of intellectual property. BUT – as others have pointed out, the most difficult part of your project will be moving it beyond outline form and nailing down plot, narrative, character development, and finding a clear and compelling voice that can speak to readers and draw them into your story.

No laws or regulations can do that for you, and it’s really the hardest part of writing anything.

Good luck.

Create a outline and mail it to yourself.

Write a draft then mail it to yourself the date will be imortant to state when you wrote it and your name on the draft
Also just put a c copyright stamp on it before sealing and mailing to yourself.
Example : my software photo Program has a watermark ( copyright) capability to add onto my created images.
I suppose that there is one such program that is out there for drafts otherwise have a lawyer draw it up for you and mail in sealed envelope not to be opened with leagal dates etc on it.
Of course have a copy of your draft to make changes throughout.

What you mail to yourself must be a copy, because you can’t open it. If you open it, there is no proving what had been inside.

The method is only useful if a case goes to court. The document would then have to be observed with name address and dates on it, and opened before witnesses to prove what is inside.