Negative interest, never heard about like this thing before. Its better to put money in vault rather than paying banks to hold money.
I think this is an idea that will become more commonplace in the future. It's the reason some economists are advocating banning cash too. Sweden and Switzerland have also experimented with it.
A Swiss pension manager figured out it would be cheaper to remove his client funds in cash and pay for it to be guarded elsewhere. When the time came to remove the money from the banks he was forbidden from doing so.
http://www.zerohedge.com/news/2015-04-25/war-cash-migrates-switzerland