Keep in mind that "proof" of rabies vaccination is a copy of the rabies certificate issued and signed by a vet, containing a detailed description of the pet, the manufacturer and batch number of the rabies vaccine given, etc. A tag is NOT proof. If you don't have a copy of the current certificate I'm sure the vet who gave the vaccination would provide you with one.
I also always bring along a copy of my latest receipt from my vet's office for each pet. On the bottom it shows when the pet is due for his next vaccinations (thus also proving what he's had and that he's up to date on them). In the event of a medical issue I figure a vet might want to see that, and in case we needed to use a boarding facility they'd likely want to see it.