Hi,
I don't know about Vermilion County, but I had a family branch move to Will County - only two counties away from Vermilion (at least I have notations of them in Wilmington) in the mid-1860s. They were in Ross County when a couple of their sons enlisted at the beginning of the Civil War, but I know they were in Illinois in 1866 when the wife died, in 1867 when the husband patented a heating stove, but were back in Ohio (Hamilton County this time) by 1868, when three daughters were married. I've oftened wondered about why they might have moved there at that time. The family of one of the daughters married in 1868 moved back there again about 1880, too.