Need to translate from Eng (US) to Vietnamese

Hello ~ Take a look here:

Vietnamese Translator Offline 9+ - App Store - Apple

Also you may want to leave feedback so Apple may include Vietnamese to iPhone translator as it is not available language at this time but this above app is very good:

How to use Translate on your iPhone - Apple Support although Vietnamese not a choice here yet. Go to product feedback and make a suggestion at below link:

Product Feedback - Apple

~Katana-San~

Hello ~ Vietnamese is not a supported language as of yet…important to leave your feedback so they know there is a need to add it.

~Katana-San~

The Microsoft Translator app for iOS and iPadOS supports Vietnamese, and many other languages, and can download dictionaries to work offline.

https://apps.apple.com/us/app/microsoft-translator/id1018949559