When the Jays call him up, I don't think he's going back down. In other words, if they call him up in Sept, it's probably to prep him to start at 3b from opening day 2019 onwards. But by Sept, he would have had AA and AAA experience at 3b everyday, rather than a month in AA and then thrust into the bigs to DH most nights.
The service time issue is only relevant if they are open to keeping him down until May of next year. Like I said, that would be my choice. It's unfair to the player but top orgs are doing the same thing so you either adapt or die. However, I don't think they will keep Vlad from the bigs for that long. So Sept + 2019 starting gig avoids super 2 issues and gives him enough minor league AB's to justify the call up.