Vapour blasting uses abrasive material in water. Less mess than dry blasting as the abrasive dust doesn't blow all around although there is some sprayed around. So maybe they use glass beads, maybe they use carborundum or whatever.
You want a mild abrasive that minimizes cutting into the alloy.
I would go with soda blasting which is very kind to the metal but strips off paint and dirt. It's safe for fibreglass as well, if the operator is careful, so pretty much doesn't cut metal at all. There's also dry ice blasting that uses pulverized dry ice, never tried it but since there's no other abrasive there is no cleanup required as the dry ice just turns into CO2 and is gone.
Anyway, yes the guides should be capped and cam bearing bore surfaces masked.
Grit will get down into all holes, I take the blasted parts to a car wash and use the pressure washer wand to flush out all the holes and the ports... a raincoat is recommended (or a bathing suit), you will get sprayback. If you have your own pressure washer then great. I then run a small tube brush in the holes and chase all threads with a bottom tap to ensure cleanliness.
