In my case, even after I realized composer update was necessary, I had trouble finding out where to run it. I had been (more or less) blindly following the steps in the "Full-Stack Vue.js 2 and Laravel 5" course when it failed at the "php artisan --version" command with "Failed opening required "/home/vagrant/projects/vuebnb/vendor/autoload.php".
I didn't have php, composer or anything installed on my computer (macbook pro), so I had to execute "composer update" inside the vagrant shell, and I had to be in the ~/projects/vuebnb directory.
I know this doesn't add to the answer ... just putting in text so that future explorers might find this answer page easier.