I did video drama last year but didn't do extention 2 english. I have alot of friends who are doing ext 2 english and are doing the video thingo and to me, the difference is that ext.2 is a bit more general and is sort of marked on how constructed your story is and how well you present it. Video DRAMA on the other hand, goes into more detail and expects you to be very competent in presenting your idea through the medium and expects you to script, direct, shoot, edit very competently.
If I were to generalise, ext2 is more the idea/concept whereas video drama is more the WHOLE aspect of short-movie making. Don't take my word on this though becuase I personally haven't gone through the actual requirements for ext.2, so to be sure, ask a teacher or go through the syllabus/criteria and so forth.