Setting your vids to Stream Only will mean purchasing Members will only be able to watch your vids through ManyVids and they won't be able to download the content.

You can set the Stream Only option to vids when editing the details of a vid. On the 'Edit Vid' page, you will see a section titled Security Options  (by default it is set to allow vids to be downloadable). Simply update this setting to  "Stream Only by selecting "Yes" from the drop down and save your settings.