what it says, what it means...
no guns allowed on company property or vehicles unless expressly permitted by law
This says to me - you may not have gun on company property, but if you are expressly permitted by law you may have them in a company vehicle. There was a VA SC case that had a similar exception clause and it was held to operate only on the last condition. The courts do not always follow common english language understanding, particularly if they "define" a term within a statute. That said...
I believe the policy is sufficiently ambiguous as to potentially provide a "first offense out" as others here have clearly shown. Courts often consider what a reasonable person of average intelligence would make of a situation, and I believe that this language is one of those situations. Others here have echoed that a permit is exactly that - permission from the state to break the general prohibition in the law. I see a few courses of action here as a supervisor or mgr.
1: Leave it be. The status quo; no written communication. If discovered, it coudl lead to an employees termination. If this happens, ethically, OP is at least partially responsible by tacitly approving or at least not erring on the side of saying no, that's not allowed. Were I OP, I would be a little concerned about legal implications of this action if one of the carriers was found out and fired. IOW, could I be sued and held liable. More important than that, I believe it would be wrong to hang someone out like that.
2: Of the motos who carry, if there is a person with a genuine, provable need, and were I OP, I would possibly consider approaching company management & HR - I have a *highly valued* employee who was involved <example> in a court case as a prosecution witness </example> they have had death threats lodged against them and have requested that the XYZ zipper corp. grant a clarification on this weapons policy and that they be granted explicit permission to be armed to & from work / the office (perhaps secure storage in the office). Basically the goal would be to get a 'case by case' agreement to respect the employees rights.
3: Use the permit is an expressed exception doctrine and tell the employees that this is what you think. This could be totally worthless unless a fired employee goes to court. A very expensive proposition and it could put OP in jeopardy for their job as well.
I believe it was leadcounsel that said the permit applies to public property. That is true, but that is not the end of the permit's umbrella of legal protection. As previously discussed the legislature created a general prohibition carrying a concealed weapon. A permit grants permission from the state to violate that law in a similar manner as 'leo in performance of their sworn duties'. IOW, you won't be prosecuted for it. All of the shall issue laws I'm familiar with make some sort of exception to this by "except where prohibited by law" or similar. In VA it says "shall not authorize the possession of a concealed handgun ... where prohibited by the owner of private property". Statutes are read very narrowly and this prohibition in particular exempts all but the owner from the act of prohibiting, and by implication allows the permit's umbrella of legal protection on private property where it is NOT prohibited explicitly. Thus, if you carry on to property that's posted, the permit is no longer valid and you're breaking the law. but, if you carry on to private property that is not posted or prohibited explicitly, you are not breaking (Virginia's) law.
IANALATINLA,YMMV, HTH