There were other Vowles (Vowels) in Illinois in the 19C, but mainly in Boone County, which is not too close to Henry County (if my geography is correct). I do know the ancestry of these Vowles's, so if we can find a link that would be good.
There was a William Vowls who was enlisted into the 9th Cavalry Regiment, Illinois 2.3.1865 as Private - lived in Belvidere. He was discharged a few months later. This could be your William (but he was in Boone County). Could you find out any more from the regimental records ?
I think what we need to do is to work backwards from what we know. You need to check the County Home records for any other information - age, birthplace, etc, and the Illinois censuses until you lose him. This will give extra confirmation of his age and possible immigration date. The 1890 Illinois census was mainly destroyed by fire, but we know where he was in 1890, so that's not too much of a loss.
On his marriage licence, he presumably gives place of birth as England. This may indicate that he immigrated when he was fairly young.
Can you tell me what happened to his other three children ?