I would like to create a portal for different institutions to which I would offre a moodle site on a subdomain . is this technically possible on a vps ?
You can install as many Moodles as you have space and resources for on your VPS.
You need to know how to set up your web server to handle multiple hostnames (what I assume you mean - a subdomain is something else) but that's not really a Moodle issue.