I don't think that there is any need to "reclaim" the address.
That second address would no longer be needed or used for anything. When you purchase the land, the two properties will join. Since there is no improvement on that second property, and apparently no plans to improve it, there would be no need for the second address. This really just seems like an issue with USPS.
Also, addresses are (generally) not legal descriptions of properties. The legal description is your plat description in the deed, which can be seen online at the Assessor's Office. Your neighbor across the street, using the wrong address, doesn't really affect you since it isn't your address either.
If you are really concerned, contact your local USPS office and ask them about it.