"I think most vests could block the penetration of a 10ga slug, but the blunt trauma force would be several inches into the vest I would have to think. Severe shock would probably break ribs and possibly destroy heat and lung tissue without actualy penetrating."
An instructor of mine told us a story about just such a question. At a school where he was teaching at the time, they decided to see if a vest would stop a 12GA slug, so they cut a piece of 3/4 inch plywood to size, fit a vest over it, and fired. I don't know the range, but it was probably close. The slug never actually penetrated the vest, but the slug/vest combo punched an impressive hole through the plywood. He came away thinking that a vest offered no meaningful protection against slugs.
That said, after firing plenty of 12GA slugs, I can't even *imagine* wanting to shoot 10GA. Geez...use a rifle.
Tim