what you need is info on that file AND what its gonna be played on, you need to match formats---->in other words, if european use PAL format, if US use NTSC, the video also will need the correct frame size, audio bitrate, video bitrate, and FPS to match the VCD format its gonna be played on, it may need rescaled and or cropped depending on what its like now.Some software will do all of this for you, some only does parts of it.The other thing is, if the original isnt very good, it wont be good when converted either.Converting a DVD to vcd isnt hard, lots of software to do that, and they usually come out nice, random other video files sometimes can be a problem.
http://www.videohelp.com good site, links to tools too, most of them freeware
Ive been doing alot of this lately, conversions take awhile too, its taken me quite a bit of work and experimenting to get things right, lots of things can go wrong, like the sound getting out of synch for example.Sometimes they are easy, if its an mpeg file that makes it easier to begin with.