Actually insuring a vacant house isn't that hard. There are riders that will do it or a premises liability policy. Note that the reason you can't find anyone to insure it is because an agent that has to work just for this one small policy isn't going to waste their time. Find a reputable agent, and give them all of your insurance (personal auto, homeowners, personal umbrella). They will be happy to find coverage for your vacant house at a very reasonable price with an admitted insurance company. Don't try and buy direct, buying direct just gets you poorly educated salesmen that can't analyze your risk properly. Go to an agent that has a CIC, CPCU, or CRM designation.
P.S. I'm one test away from a CIC (Certified Insurance Counselor) and I just finished the test on Personal Insurance today :-)