A HTTP error is decided by and only by the server application. Here, error 403 is generally returned to indicate that the user is accessing a resource for which it does not have the rights.
If the resource has already been accessed (the video runs in the page), it could be that the 2nd attempt to access the resource (this time by VDH) is not allowed. Or maybe they implemented some sort of protection mechanism that requires a specific HTTP header in the request. Anyway, i'm afraid VDH won't work here.