Steps to reproduce: 

1) Obtain a complete list of Vietnamese characters. Either visit https://vietnamesetypography.com/alphabet/ or copy the below:

ÁáÀàẢảÃãẠạĂăẮắẰằẲẳẴẵẶặÂâẤấẦầẨẩẪẫẬậĐđÉéÈèẺẻẼẽẸẹÊêẾếỀềỂểỄễỆệÍíÌìỈỉĨĩỊịÓóÒòỎỏÕõỌọÔôỐốỒồỔổỖỗỘộƠơỚớỜờỞởỠỡỢợÚúÙùỦủŨũỤụƯưỨứỪừỬửỮữỰựÝýỲỳỶỷỸỹỴỵ

2) Paste the characters into any text field in Minecraft (chat, book and quill, etc.)

3) Observe that while most characters are supported, a few are not and default to Unicode, specifically: Ẵ ẵ Ẽ ẽ Ễ ễ Ồ Ỡ ỡ Ữ Ỹ ỹ