I have enabled External Player (VLC-2) for my .flv trailers and it works fine, apart from when it returns to MB.
In NEO and DIAMOND themes, if I goto RECENTLY ADDED movies, select a movie, and then play the trailer, VLC plays it fine and returns me to the movie details page in MB.
So far, so good.
If I now press BACK, to return to the list of RECENTLY ADDED MOVIES, I am returned to the movie VF instead.
As this happens in all of the themes, I believe this is MB core, unless this is just a limitation and not a bug.