It the objective is to provide the biggest boost to the website's ranking (not the individual video), is it best to embed the YouTube video or can you post the video on both YouTube and Vimeo and embed the Vimeo video for the benefits previously described?
My gut feeling is that Google would see the direct linkage between the video on YouTube and the website it's embedded on and potentially that would have (albeit probably small) more benefit in the website's ranking than embedding the Vimeo video?
Is the SEO claimes on this Wistia page true? Could I then use Wistia for embedding and then also post the videos on a YouTube channel for maximum exposure?
Vimeo, like YouTube, is a powerful domain for SEO, but when you host your videos on these platforms, you are not doing your website's SEO a favor. When you upload your video to Vimeo or YouTube, the search engines are indexing the original url, not your website's. In contrast, when you embed a Wistia video on your website, your website gains all of the SEO benefits.
Vimeo videos are ranked to drive traffic to Vimeo to keep users on their platform. Wistia videos drive traffic to your website--not ours.