Vietnamese alphabet is not fully supported by Minecraft font
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