Our rental car in France does not have the items required by law, such as a florescent jacket and a red triangle. Should the car rental company provide these items?
It would make sense but I could not find any authoritative source about that. The problem is that I don't see how their failure to provide these items would free you of your obligations in case of control (not to mention protect you from an accident should you find yourself in a situation where you actually need the safety vest). Right now your choice therefore basically seems to be between walking out/returning the car, buying safety vests yourself or risking it.
Even if the courts were prepared to recognize that the rental companies are contractually or legally bound to provide them, I guess you mostly want to continue your trip as soon as possible and you cannot hope much from litigation so whether they should do is less important than what they are effectively able and willing to do.
Did you try talking to someone at the rental company about the issue? It might simply be a minor oversight or theft from the previous renter. Just as with scratches and the like, reporting the problem as soon as possible would also make it more difficult for them to pin it on you later on.
Lots of semi-official information on the rules governing car rental in France can be found on the DGCCRF website but unfortunately nothing on additional safety equipment.