In Hong Kong, the concrete walls of our vault were reinforced with "tang bars". They are a series of twisted steel metal strips of a hard, tough metal - they come in sheets, I think. They make it very difficult to create a hole in the concrete. They are not cheap.
http://www.google.com/search?q=%22tang+bars%22+vault&sourceid=navclient-ff&ie=UTF-8&rlz=1B3GGGL_enUS259US259
http://www.google.com/search?q=%22tang+bars%22+vault&sourceid=navclient-ff&ie=UTF-8&rlz=1B3GGGL_enUS259US259