I have no trouble at all with my VST ZIP drive on my second generation PB 233/512/160MB/6GB/OS 9. (I did start experiencing "The Battery Problem" but the original "fix" seems to work.)
I used to have trouble waking up my PB and not crashing, even on OS 8.6, but 9 solved that with a clean install.
You might try hot swapping between your CD and Zip. I always change them "awake" and I found that if I do it while in THE FINDER I never crash. I started this in OS 8.x and continue without a hitch.