I found this great article from Larry Higgins on https://www.biggerpockets.com/ on How would you skiptrace an owner of a vacant property?

Vacant houses are great leads, especially truly distressed vacant houses, but the owners of the properties can be hard to reach sometimes.  But the effort you put into finding them is worth it.  My general rule of thumb is the harder they are to find, the more I like the lead.

When you skip trace the owners, pay attention to all of the information you get back.  You'll find some of the owners are deceased, or maybe very elderly.  In these cases you'll want to contact their relatives which are generally not too difficult to find with a good skip trace service.

Other things to pay attention to are address history (living out of state?) and financial distress indicators (bankruptcies, liens, judgments).  This info can help you prioritize leads when you have a lot of them or at a minimum give you good information on each seller and how motivated they may be.

I also agree with Sean that calling is the best way to contact the owner is calling them.  I say this for two reasons:

1.  If you mail the owner, you never really know if they get your letter.

2.  They may get your letter, want to sale, buy not call you because they think they can't sale the property for some reason or in some cases may not even think they own it anymore.  If you call enough vacant properties, it's not a matter of if, but when you encounter this.

Also, don't hesitate to text the owners or their relatives if they won't answer or won't call you back.  Even if you're not sure if you have the right number.  This will make a difference in whether or not you get some deals.

I'm not opposed to mailing, I just wouldn't rely on it.

Vacants are great leads......GOOD LUCK!