Miggy didn't play 3rd base full time till his 3rd season with the Marlin at the beginning he mainly played outfield . He moved to 1 base when he was traded to the Tigers then moved back to play 3rd to make room for Price Fielder then back to 1 base with Fielder retiring.
Vlad Guerrero jr is a big man listed at 6'2" 250 lbs it takes a lot more work to keep a big body agile and athletic. The fact that I don't like he's already shown a vulnerability to injury with this oblique strain possibly due to some conditioning early on his career. Vlad is possibly one the best hitters this game ever seen in longtime so why wouldn't the Blue Jays want to protect this kid from possible injury risk to insure him of a healthy longest producing career.
Here 's the scouting report on him" At third base, Vlad’s no butcher, but he’s still average-at-best defensively. His offense will certainly cover that, but it is entirely likely that he will end up moving across the diamond or even to an outfield corner to utilize his arm. His quick feet and soft hands could be a big asset at first base."
The scout clearly think Guerrero jr would best suited to play 1 base.