Hello Jamie,
You can make the certificate into a PDF, download and then send the PDF from your email (or in the Cvent email by uploading the PDF in
Admin > Libraries > Documents > in the Event Email [under Content & Insert tab]
click Insert Document and find the uploaded PDF).
Click on the following link to see how to download the certificate in PDF (
https://cventhelp.force.com/apex/CommunityArticle?id=000001794).
I hope this helps.