Vapour blasting is probably kinder to the surfaces than conventional shot or bead blasting, but yeah, the stuff that gets left inside once the process is complete is very fine. I've had three engines vapour blasted recently, and no matter how hard the guy who does the work tries to clean them out, I still find traces of the bead/water mix in little places where you really don't want it. The problem is that the mix is a bit like wet sand - and has the same adhesive properties.
I tend to do a couple of things after my stuff has been blasted to try to get rid of any remaining crap - first, I then leave the parts in a warm-ish, dry place for a couple of days/weeks to allow the water in the vapour mix to dry out. I then clean out/ream all of the various threads on the cases (you'd be amazed how much crap gets trapped in them and then blow through the oilways and other key areas with compressed air. After that, I flush the parts with some kind of solvent-type liquid - last time I used turps; the one I'm doing now is going to be washed out with parrafin to see if one's better than the other. I've tried the dishwasher route before, but unless you're using a dishwasher with no salt in it, the parts will gather salt deposits (and in the cylinders, start showing up surface rust) before the wash cycle has finished.
Mine seem to have come out of all of that pretty well, and I've not seen any traces of anything going bad with the newly built engines. Incidentally, I had a set of 750 carbs blasted recently and the bodies now look like new. Just make sure that if you're not going to paint the cases (I don't - never found a paint that's durable enough), coat them in ACF-50.
The alternative is to find a furniture/metalwork restorers and have them dip the cases in acid. It takes about a week, does much the same job as a vapour blast, but doesn't leave any residue.
Hope that helps.
