Setup Vietnamese Input with fcitx5

   
   sudo apt install fcitx5 fcitx5-chinese-addons fcitx5-diagnose
   

Additionally install some CJK fonts

 sudo apt install fonts-noto-cjk fonts-noto-cjk-extra
 sudo apt install xfonts-way

then update the font cache with

   
   sudo fc-cache -f -v

Add the following environment variables

#~/.profile
export XMODIFIERS=@im=fcitx5
export GTK_IM_MODULE=fcitx5
export QT_IM_MODULE=fcitx5

Check the setup:

fcitx5-diagnose

Change default input method from ibus to fcitx5:

 sudo apt install im-config 
 im-config -c 

This adds the following configuration in .inputrc r .xinputrc

 run_im fcitx5
 

Select input method:

 fcitx5-configtool
 

Open a GUI application, gvim for instance, and click Ctrl+Space (or use language switcher top right) to switch from English input to Vietnamese input.

Google search shows, to support Wayland composer, .config/environment.d/profile.conf needs to be updated with the following content

 XMODIFIERS=@im=fcitx5
 QT_IM_MODULE=fcitx5
 GTK_IM_MODULE=fcitx5