seems like plastic
The vault plate is reported by some to be used to take some of the unpleasant vibration out of the cue shaft. I think it is Predator that explains the construction of the REVO in great detail. They can't seem to make up their mind if the vault plate is a ferrule or not though.
My impression is that the vault plate is made out of something that deadens the hit not something superhard or heavily reinforced to pass through every little vibration. I think it does have a fiber pad protecting it supporting the idea that it isn't real strong. Probably a medium strength and hardness plastic but that still leaves a lot of options. Looks like it is probably injection molded which may eliminate some options.
Hu