An easy way to display external subtitles with DVD movies in VLC, is to copy and rename the subtitles after every VOB file of the movie (usually VTS_01_?.VOB). So you would end up with several copies of the same subtitles file, named something like VTS_01_1.LANG.srt, VTS_01_2.LANG.srt, VTS_01_3.LANG...