Mailing to Vacant Houses / Follow up to Driving for Dollars

[Vlad Kuzin]

I went driving for dollars in the neighborhood of my interest and collected addresses of 30 houses. After I've mailed letters to these addresses, 8 of them came back as "vacant" or "unable to forward" or "attempted - not known", etc... All of the addresses for these properties came from county tax assessors office. I've left my post card by these houses (while driving), however how can I follow up with these leads? Is there is a better way to track down these owners? Is there is anything I can put on the address block which will make USPS deliver even if house is vacant?