well you questions could contain many answers, you should be more specific my friend. but here are some thoughts about video quality to take into consideration. If you are ripping and converting to vcd, i think that one would convert from vob to some sort of avi file then to the mpeg1 format. If you are converting to svcd you couls do the same as above or as i do sometomes, i use the free tempegencoder and convert from "ifo" to svcd. i do not recommend using the ifo to convert to vcd though, being that quality can be reduced more. most people prefer to use svcd becasue the bitereates are better (thus the quality is improved) but unfortunatly some dvd players will not properly play svcds, so im stuck with vcd. for vcds i recomend getting an mpeg encoder such as tmpeg and for converting to avi i recommend either flask mpeg or vcd galaxy. convert to divx mpeg 4 (low motion) at a high quality and change your configurations as you wish. save as avi then convert to mpeg. if you do it right, you should get nice quality. check out the how to guides here at after dawn, this site rules.
If you mean re-encoding from the original source, that depends on how you encoded originally, and what's wrong with it. If you mean improving the quality without going back to the original source, you can't do it. What you may be able to do is trade imperfections that really bother you for ones that don't bother you as much, but if the file wasn't encoded well originally, any program that you use to "fix" it will either be guessing what information needs to be changed or just hiding things, either one of which can make it look worse. Once again, though, you haven't said what's wrong with it, so nobody can really tell you how to fix it. Your question is kind of like saying "My car doesn't run right. How do I fix it?".
Another point, whenever you encode something that very best you can do is maintain the quality. As a general rule: "poo in = poo out" so it all is determined by the input quality.
So basically my understanding is video quality can be maintained but not improved....as Praetor said poo in=poo out. thanx to every1
