Yes, 3rd pairing. The opportunity he was given by the coach as a young defenseman. Still more impressive than anything Theodore has accomplished.
And I'm not saying Theodore is better than Vatanen.
What I'm saying is that Sami still has some of the same concerns that you're placing on Theodore. As a 3rd pairing defenseman, Boudreau can use Sami in ideal situations, putting him out there against weaker competition, and making sure he isn't in key defensive situations. In other words, he can put him out there to move the puck, and create offense, which is where his strengths lie.
As a top 4 defenseman, that becomes much harder, and as a top pairing defenseman it becomes virtually impossible. The more responsibilities a player has, the more the cracks can become big gaping holes. All that stuff about driving possession, and making the players better, that's great and all, but the context is important. He's able to do that, in part, because Boudreau is smart enough to put him in those situations. The stats don't show that he has more potential as a top pairing guy than Theodore, or that his size isn't, or won't be, an issue. What they show is that he's an effective NHL player, when used in this fashion, and he absolutely is.