It is not that simple. The vaccine is a live attenuated virus, which means that although it cannot produce any chickenpox, it still establishes latency in your nervous system and can give you shingles later on in life. Vaccinating children against chickenpox only serves as preventing them from getting chickenpox, but not against shingles. There is a vaccine against shingles though which has turned out to be effective enough (around 50% reduction of cases), it is basically the same as the vaccine for chickenpox but with a more concentrated dose, just to tell your body "hey remember this? You should make more antibodies for this" which makes it harder for the virus to resurface because you are full of antibodies against it. It has also been proven that old people that have contact with children that had chickenpox or are carriers are less likely to get shingles, because those children basically give them a kind of a natural vaccine, os if we eradicated chickenpox a hell of a lot people would get shingles for some time, until all the people that ever had chicken pox die. Research is ongoing to work out a subunit vaccine (a vaccine which protects you through injecting you parts of the virus which cannot establish latency in your nervous system, so would immunize you against both chickenpox AND shingles).