Depends what you mean by "poor quality".
If you mean, excessive pixellation especially in areas with a lot of change in the image (eg much movement, or soft crossfades) and/or a lot of detail - then the cause is the restricted data rate for VCD - 1150kb/s fixed. The answer is in the mpeg converter. They do vary hugely.
The best results I have ever seen have been from a free converter called tmpgenc - get it from
www.tmpgenc.net . Use the highest quality (very slow) setting and be very patient (leave it to run overnight if necessary - the program has a "shutdown when complete" option). This converter, given a "clean" source, will produce a fully VCD compliant mpeg file almost free of visible compression artefacts - as good as it gets.
If you mean, poor definition/lack of detail, then, with VCD it's unavoidable. The VCD format for moving images has a pixel definition of 352x288 for PAL, 352x240 for NTSC, which is half that of DVD in both directions. This is roughly equivalent to the definition available from VHS.
Depending on your player, you could try making an XVCD or SVCD - these are "unofficial" extensions to the VCD format with higher data rates and/or better definition. tmpgenc will make suitable files, and Nero will burn them for you. But not all players will play them. And file sizes are larger, so you get less on your CD. Check out
www.vcdhelp.com for more details and player compatibility.
VCD stills on the other hand....
a) don't suffer pixellation because there is no movement in them so they aren't constrained by the VCD data rate.
b) have twice the definition in both directions; 704x576 PAL, 704x480 NTSC - the same as DVD.