Super late to the party, but I finally upgraded to a new Mac and initially ran the same global install command, which resulted in the same issue.

So if anyone new has the same issue and reads this, you can fix it by simply adding the composer path to your .zshrc file, which Mac uses by default.

In my case the path to composer path is :

    export PATH="$HOME/.composer/vendor/bin:$PATH"

Open your .zshrc file by running :

    sudo nano ~/.zshrc

Add the path to the file and restart the terminal or run:

    source ~/.zshrc

The you should be able to run :

    vapor --version