Ok, I think I should elaborate on my previous comment. You seemed defensive. First, you can ask anyone on this forum if printer model and settings have anything to do with print results and the answer will be 'yes'. Maybe your model is not rated for adhesive vinyl and that may produce undesirable results. Or the settings can be causing the yellowing. I know on my old 252, I had to run adhesive vinyl on the heavyweight setting. If I used the coated setting, the color would wash out. Weird, I know. On my new C75, I haven't discovered all of it's quirks yet. We got three jobs to do this week on adhesive vinyl so I'll make a note to try the coated setting to see what it does.
As for bashing the material manufacturer? I think you are being overly cautious. As of now, you're not sure if it's the material or the machine. If it is the material, it's not their fault. It just doesn't work on your machine. Nothing wrong with that. Heck, maybe they'll see your post and offer you some free samples and advice to try it again?