What font editor are you using? Is your font editor capable of generating variable fonts? If your answer to that is no, find one that does.

BTW what kind of changes do you want to do that you need to have separate styles. The font supports VN codepage so I assume it contains a complete VN specific characters.

There are 14 different installed fonts in Windows 10 with the file name Bahnschrift, and all have the same date last modified.

The file name and the font name are two different entities that often, but nor always, have the same name. The font name is set internally within the font. The file name can be anything, or altered with right click -> rename. Renaming an installed font will render it unusable.

This is how I would extract the individual styles:

1) Create 14 separate new subfolders inside a new folder. Each of the 14 subfolders would be named/numbered 1-14.

2) Open C:\Windows\Fonts, and using Windows search, search for Bahnschrift. One result will come up in search results.

3) Right-click -> copy the search result.

4) open the first of the 14 subfolders.

5) Right Click -> Paste in subfolder 1. When prompted with a dialog box asking what you want to do with the 14 files that have the same name, check the first box only.

6) open subfolder #2.

Repeat step 5), but check only the second box.

7) repeat this procedure for font and subfolder #'s 3-14. This should give you all 14 styles, each in its own folder. At this point, you can alter the file names to match the internal font names.

I didn't test this, because I don't use Bahnschrift, but I'm confident that this method would work.

It is a variable font that is why it is a TTF.
Why do you need to have a separate font for each style when you can have all the styles in a single font?