Like anything in restoration it is in the preparation that the final results will be either great or poor. It takes up huge amounts of time, but when done right , it ends well.
I seriously doubt they will simply vapor blast the cases without them being bead blasted first. The vapor blasting simply does not have the strength to remove difficult stains or similar whereas the bead blasting will. In any case I strongly suggest you remove the pump sleeve, you will find it extremely difficult to successfully mask off the timing side area enough to stop grit (either from beads or the vapor blasting medium) getting in behind the upper cavity beyond the pump sleeve. I'd hate for you to make an expensive mistake..........