Rebuilding the font cache not only takes time, sometimes it can make VLC hang completely, which can get old quickly. To make it stop, just open up VLC and go to Tools > Preferences. Hit the “All” radio button in the bottom left hand corner, then go to Video > Subtitles/OSD in the left-hand pane. In the Text Rendering Module dropdown, pick “Dummy Font Renderer Function” and hit save. The next time you open VLC, you should be free of that annoying cache-building dialog.