Microsoft recently released the updated version of its Skype app on Windows Phone 8. Skype can now be used for sending video messages to your friends. It is truly very late but still the app is getting all major updates which it already have on all other mobile platforms like android and iOS.
You can add video message directly from where you use to type messages. A '+' button on the left side of the keyboard lets you add participants and video message. It looks like shown in the picture below. You can record video message for maximum of 3 minutes.