How do I set Siri to speak Vietnamese on iPhone?

Using Siri for Vietnamese:

You Can’t. According to Brave, what you are looking to do is just not an option —and

such is the case.

So, Recommend this to Apple:

Do your Part: Report this to Apple. Apple may not get back to you directly, but the more feedback Apple receives on this, the more they will know what is their customers prefer, and what and what features to include in updates to come. Apple will know nothing of your thoughts, if you don't provide them with them.

1. Go Here: Feedback - iPhone - Apple
2. Select: "Feature Request" for the "Feedback Type"
3. Comment: that you want this to be an option and why. Include the URL to this thread.
4. Proceed from there as necessary

***Screenshot:

Apple publishes Siri language coverage at

iOS and iPadOS - Feature Availability - Apple

and it is not yet on the list.

Vietnamese is however available for spoken content, translation, dictation, system language, keyboard.