I know that, i have a flash plugin for the browser
Well,my apologies.Had you made that clear in your first post i wouldn't have suggested it.
I've got a couple of .flv files from Youtube,both (s)mplayer and Vlc play them just fine.
You say you have Vlc installed,yes?,try playing the file with Vlc from the command line:
Something like;vlc <yourfile.flv>
.See if you get any error messages that way.