You guys are making this way too difficult. It’s very simple. When you deviate in VIPS, if you have multiple legs and click to deviate on both legs, that’s it. You are deviated for both legs. You can’t undo this. This will cancel the company provided “Scheduled” DH ticket. Even if you buy a ticket for a seat on the same flight that your pairing shows, you are still technically deviating. Just because you have chosen to take the same flight that shows on your pairing, doesn’t make it a “scheduled” DH. You gave up the scheduled DH when you clicked deviate in VIPS.
Now, if you only click to deviate on the first leg of the above scenario, and keep a company provided ticket for the second leg, you can then do your final check in and no longer be deviating. If you are doing your final check in when you have deviated on both legs, assuming you are more than 100 miles away, that would be improper, as you are still technically deviating to your assignment.

Edit for spelling error