I say an article in the NY Times last Sunday that answered the question but was not area specific so I asked my insurance guru buddy Theresa Byrnes the question. Here is her reply:

“ Yes, I read the same article and yes this is true.
The reason it is more expensive is because first of all, most insurance companies (Farmers included) will NOT write vacant homes.  We have done this, but they can not be vacant for more than 90 days and in the market today it can be tough to flip these houses.  So….then you are dealing with “specialty dwelling” coverage which I can write though Foremost (Farmers affiliate)…however…the premium is higher.
 
The reason for this is because vacant homes are risky.  Vandalism is very high and water damage also is very high. When pipes break no one is there to know about it so the water runs for days causing catastrophic damage to the property. 
 
I get phone calls all the time from Realtors and attorneys or ask me to help them with this because foreclosed properties can’t get insured because the owner left and cancelled the insurance policies, and now it is vacant.  Because of all the ridiculous negative media about the housing market….it’s raising more concern….
As far as how much this can effect your premium? It can double. I have been writing Foremost policies more now than ever because of the exact thing the paper wrote about. So….for once, the paper is right.
 
I hope this answers your question and prevents any problems in the future.”

Theres Byrnes Farmers Insurance