We used Vidyard as well in a different way. Instead of putting the Marketo form inside the player, we created a landing page with the form and the thank you page contains the On Demand video. We can always advertise on our website the registration page for each On Demand. Either solutions works. it depends on your needs.
If you use On24, you can get this done within the tool and customize the page within ON24. The tool will pass back the stats on how much the On demand has been viewed back to Marketo.