I'm not a huge Slayers collector, but I have seen 
first-season Zel and Gourry without noticeable 
fading.  I suppose it is inevitable that it will 
happen eventually, but perhaps the cels you saw 
were exposed to bad conditions at some point and 
that hastened it.  Now, try finding non-faded Rose 
of Versailles cels...sigh.

OT:  MediaBlasters (Animeworks) are releasing 
Rurouni Kenshin TV on DVD in the U.S., uncut and 
subtitled/dubbed.  ADV is releasing the RK OAVs 
and movie in the U.S. with the title "Samurai X" 
on both the dub and the bilingual subbed DVD (the 
DVD has a reversible cover to switch to Rurouni 
Kenshin).  It's not related to the "Samurai X" 
hacked TV dub that may be what the VCDs you saw