If you are having trouble finding the sellers of the vacant house, a strategy I have had success with is talking to the neighbors. This is how I found the sellers of the house I currently live in.
The house sat vacant for two years after the homeowner passed away. The heir was the ladies mother who lived out of town. Luckily, the neighbor across the street mowed the property for a while and had the contact information of the owner.
Even after contacting the owner, it took nearly 8 months before I was able to purchase the property. The owners kept putting it off until the house was about to foreclose. Because I was persistent and followed up every month, I was able to purchase the property.
More often than not, the neighbors seem to know the situation with the house or have the contact information of the seller.