How can I track conversions for an email directed to an on-demand webinar Wistia page?
Depends.
With the (extra $) Marketo-Wistia integration you can straightforwardly embed a Mkto form in the video player itself.
You can do this and lots of other things even without the integration by using the Wistia API, but you will need a skilled developer who understands Marketo (Munchkin and Forms APIs).