I've always heard people bash Armor-All, though I've never heard an explanation as to why or what it actually is about it that is bad. That being said, I've used a leather conditioner to help keep these style dashes soft. Meguiars seems to be the best even though I'm not a fan of anything else they make.
Is your dash N.O.S. or actually a mint/nice used unit? N.O.S. means it's brand new in the package/box, or was. It also usually refers to a part that is old and for a vehicle in which the part in question hasn't been produced in a while, and has been sitting at a dealership parts dept. for a long time. This is where the "new old stock" term comes from. Actually, the "Vintage Parts" system has pretty much taken most of that out of the equation as they buy out all the old remaking stock from companies like GM and sell all remaining quantities until depleted.
Anyway, the reason I ask is that sometimes old original used parts, or possibly even N.O.S. stuff may have become brittle over time without actually appearing to be. By putting anything on them might further weaken the material, in this case the vinyl, and end up making it crack. A good way to tell is if the surface is sandy feeling and/or slightly flat/dull looking. Also, the material might still be pliable if you squeeze or push on it, but it will be tight and not give much. The original dash in my Regal is still nice and "squishy", so it's fine to condition it to keep it that way.