cleaning up the slant six head

This what you're describing doesn't add up. Vinegar is a very (drinkably) weak solution of acetic acid. It isn't a good choice for cleaning engine parts, but only because it's not strong enough to do much of anything for you but waste your time, not because it "softens cast iron", which…um…it pretty much doesn't. Especially not selectively on only the machined surface; vinegar's just not quite smart enough to tell which are the machined surfaces. I won't quite say "I haven't seen it happen, therefore it didn't happen", because that's rude and unhelpful, but I will say I think you aren't seeing what you think you're seeing. I've seen an awful lot of hard-cooked gasket material that looks for all the world like it's part of the metal until it's softened enough to scrape off, whereupon it looks like you're scraping metal off. I could be wrong, of course, but use a proper transverse scraper (not a screwdriver) and a spray can of gasket remover, and I suspect with half an hour's work you'll be looking at a completely un-"softened", flat machined 100% metal surface right where it's supposed to be. I'm also curious what the other machined surfaces (valve cover gasket rail, for example) look like.