To add Vector's SiteID pixel to your site via Google Tag Manager:
Login to your GTM
Select your account / container
Click "Add Tag"
Name your tag "Vector"
Click "Tag Configuration"
Select "Custom HTML"
Paste your install code into the plugin and deploy
Click "Triggering"
Select "All pages"
Click "Save"
In the top-right corner, click "Submit" to publish your changes to production