limiting viewing time of videos on vimeo (Cutting the video short) on ASP.NET site

A customer has a site written in ASP.NET and C# (.NET4 VS2010).
I have been asked to link videos into some pages. The videos are hosted on vimeo and are flagged as private so general public can not view them (on vimeo.com). The user has stipulated that if an unregistered user (unregistered on the customers site) views a video they should only see the first 5 minutes. After those 5 minutes they should be prompted to register or log in. I'm fine with forms authentication in ASP.NET.

At this stage I do not know what type of account the customer has on vimeo, I am waiting for that information.
I do not have any code examples to add.

Ideally I need to prevent users from downloading (either directly or by some other way) videos maybe by obfusticating the actual urls in some way.

What I was wondering is if anyone has tried controlling the length of a video being viewed from vimeo, especially in the registered/unregistered user context on an ASP.NET site.

No i never done something like this but you should provide some more info if someone is to help you.What are u using for video viewer for example.

I have realised 2am is not a good time to be asking questions on forums.

I have added additional information which hopefully better explains things.