Pox confusion

I actually had time today to get a few things done. The truck was in bad need of some routine maintenance so that was first on my list. No sooner had I arrived at the shop than Mr. Perfectly Stubborn started calling. When I finished talking to the guy about the truck and called back he told me that he had to go pick up Jo. He had broken out the night before on his neck and back. We had decided that a bug had got in his shirt and bit him a bunch of times. It looked a little worse this morning but I gave him some Benedryl and put Caladryl on them and sent him to school. They were now itching so bad that he couldn't stand it and needed to be picked up. Mr. Perfectly Stubborn was perfect today and took him to the doctor for me. Despite the fact that he has had his vaccination he has the Chicken Pox. I'm confused, why do they get the shot if they can still get chicken pox after having it? They say that it will be a milder case - we shall see. I've been reading about the vaccine since I got back and I am left with a lot of questions. They aren't sure how long the effectiveness of it lasts and if I remember right it is worse to get it when you are older. I'm wondering how this effects getting shingles when you get older too. All of my kids have had their shots for this but now I'm wondering how smart of an idea that was.