Centos-Varnish-Virtualmin-DomainCreate-Composer-LARAVEL Installtion Guide with Sequence Required*

I don't have any direct experience with Laravel, but here's some pointers for genera similar cases:

Virtualmin should come before installing any additional packages, and definitely before enabling any third party yum repos (and third party repositories should be enabled with extreme caution). We have tested with EPEL and SCL repositories enabled after install, and that's generally fine. Other repos will need you to test them yourself before relying on them in production.

You should always perform actions for a domain as the domain owner user. Whether you ssh in as the domain user, or su from root, all of the files for the domain will need to be owned by the domain user; otherwise, Apache suexec will refuse to run them (and programs that update themselves or modify data files on the filesystem won't be able to).

Virtualmin's environment is not abnormal...it's pretty much what every shared hosting account using suexec would look like. So, instructions for things like Composer should Just Work (I use composer on our systems, sometimes...I just followed the instructions for installing it and it worked fine). Just make sure you are using the PHP version that Virtualmin is configuring for use when running any Composer, PEAR, etc. commands, so that the right stuff gets installed.