I solved the same issue you have (or had), I don't know if you already solved it but here is how I solved it.
using which vue
I saw where vue was installed.
christianjavan@rog:$ which vue
/usr/bin/vue
Then I deleted all vue files inside the installation folder
christianjavan@rog:/usr/bin$ sudo rm -rf vue
christianjavan@rog:/usr/bin$ sudo rm -rf vue-init
christianjavan@rog:/usr/bin$ sudo rm -rf vue-list
Then I did exactly what it says in this link.
christianjavan@rog:/$ mkdir ~/.npm-global
mkdir: cannot create directory ‘/home/christianjavan/.npm-global’: File exists
christianjavan@rog:/$ npm config set prefix '~/.npm-global'
christianjavan@rog:/$ export PATH=~/.npm-global/bin:$PATH
christianjavan@rog:/$ source ~/.profile
Then I tried to install the new vue cli
christianjavan@rog:/$ npm install -g @vue/cli
Then I did vue --version and finally got the 3.0.3 version installed
christianjavan@rog:/$ vue --version
3.0.3
I really hope this helps.
NOTE: After trying to run vue -V again later, it did't found the vue command, so I exported the new vue directory to the PATH. When you run npm install -g @vue/cli
the command gives you the new vue directory.
christianjavan@rog:~$ npm install -g @vue/cli
/home/christianjavan/.npm-global/bin/vue -> /home/christianjavan/.npm-global/lib/node_modules/@vue/cli/bin/vue.js
You add that directory to your PATH.
christianjavan@rog:~$ export PATH=$PATH:/home/christianjavan/.npm-global/bin