Hi Shari,
I apologize for the delay in responding to you; somehow I missed your response :(. Unfortunately, there is no direct way to accomplish this objective for the reason I mentioned above.
What I was thinking about in terms of possibilities was using an iframe on a Marketo landing page or a webpage on your own website to present the Cvent registration page. This would give you tracking information the page visit. I attempted to walk through your sample Cvent registration process but was unable to complete it. That said, if Cvent provides a way to redirect the registrant to a page you control upon registration completion, you would then have the delta between people who visited the registration page and those who completed the registration process.
If you have a web developer on staff or someone you consult with, I would recommend having a cursory conversation about your objective with that resource.