John Winchester could never have been Michael's true vessel during the apocalyptic fight between Michael and Lucifer.
He NEVER could have been the vessel. Why you ask?
Because it was supposed to be a story of two brothers. Adam only technically worked because their plan to trap Dean didn't. And only because they were brothers.
Demons lie. And I am certain that the story he told Dean about thinking John would be the one to break the first seal was simply that. A lie to get Dean riled up.
That doesn't mean I think John didn't endure the torture during his time in hell. But I don't think he was offered the chance off the rack like Dean was. The chance to break the seal. Because the needed Dean to do that for them. Whatever man broke the first seal would be the one to end it all...