The materials that people most want to vitrify (fission products) mostly decay in 500 years in which case it's not such a big concern that the glass lasts for much more than 1000 years.<p>If you are storing plutonium on the other hand (as exists in spent fuel rods) the half-life is 24,000 years and the problem is much worse. On the other hand, plutonium is a good fuel and can be consumed instead of thrown out.