Is there a text version of the video? Is there are corresponding "textbook" that has everything that is said put into a traditional text version? Do you give the students the choice to either read the material or watch the video? Is there some benefit to actually watching the video as opposed to just reading an article about the material?
The reason I am asking is that I am also assigning videos to my students, but each video also has a corresponding "textbook" chapter. If the student wants to just read the textbook and looks at the examples in the textbook, then personally I do not really care if they watch the video or they don't watch the video. I just want them to absorb the material and demonstrate that they know the material through their homework assignments. It does not really matter to me how they absorb the material.
Unless it is something where the material really can't be presented in a traditional paper version, then I do not think that watching a video should ever be a requirement. If there is something in the video that can't be presented in the same way in a paper version (for example, listening to the original "I Have A Dream Speech"), then you need to explain in the description the benefits of the video over just reading the text, and why you believe the video is a benefit over the just reading the text.
How many times do you personally go on a news site and just read the text of the video as opposed to watching the news article? I almost never watch a video on a news article site. I am just saying this to present a counter argument to requiring the student to watch a video.