I do got a headache on the SEO for my company website lately. We are a marketplace for designers and makers in Viet Nam. However, our target audience is the foreigner so our primary language is English and it will be translate into Vietnamese. The point is lately we want to focus on the local and the local SEO so we have the intention of doing the reverse as making Vietnamese as the primary language. So the point is how it's gonna hurt the SEO and how what is the risk if we do that?
You can take a look at our website here: https://sonice.vn/
Your Top level domain is set in Viet Nam already. So you can put your SEO in Vietnamese. Since english would be the alternative language, what you could do is create an extension for english such as https://en.sonice.vn or https://www.sonice.vn/en/ depending on your preferences.
You would then need to create an argument to help Google redirect the english speakers to your english section
<link rel= "alternate" hreflang= "en" href= "https://en.sonice.vn" />
<link rel= "alternate" hreflang= "en" href= "https://www.sonice.vn/en/" />
https://en.sonice.vn is more practical for google:
you could say on https://en.sonice.vn
<html lang= "en" >
and on https://www.sonice.vn
<html lang= "vi">
all pages in sonice.vn would need to be SEO in Vietnamese and all those on en.sonice.vn would need to be in english.
I hope this basic overview helps out a bit