There are at least several different shades of gray satin commonly available.
I just found some really good polyester stuff that I think is perfect. It's washable, it's fairly tough, it has a very slight sheen, and it's a really pale gray. I'm using it to make hoods now, but I'll probably save enough for at least one vest. I found it at JoAnn's, but the trouble is that it's one of those things where they order a big box of "assorted bulk" fabric. Nobody knows what this stuff is, and I've never seen it before.