The issue with calling up Vlad is if the team is out of the race, there's no reason to call him up and start his service clock, and if the team is in the race, who does he replace on the big league roster? If they are trying to get him to become a competent 3B, then he's likely better off playing 3B somewhere rather than DH'ing because Morales sucks. In the latter scenario, the Jays would be better off trading some 40 man roster clutter for a mid season rental than promoting Vlad so early.
To me, it's becoming more likely that Vlad is the starting 3B in 2019 after JD bolts. His bat is damn near ready now, and one more year of seasoning might be all he needs. For service time issues, I'm being greedy though. Call him up mid-season 2019. Unfair to the player, but dammit I want Vlad in a Jays uni as long as possible!