Mike, although I never tried the aftermarket driprails myself, like most repro. parts, they usually are sub standard.
As far as being a difference between vinyl and non vinyl trim being different, I am positive they are. A few months ago I just went through this. I have a set of NOS driprails I bought over 16 years ago that would not fit my driprails, no matter how hard I tried. (of course without beating them on with a hammer) I ended up finding a nice used set from a vinyl top car and they just snapped on, no problem. Before I installed them though, I compared the two and the used GM set, definitely had a bigger lip that wraped around the driprail than my NOS ones.
Do yourseld a favor and find a set of used GM mouldings, just make sure they are from a vinyl top car. You will save yourself a lot of time and money in the long run.
Mike