I think Theodore will be better than Vatanen/Fowler from what I have seen
You do realize that, at the same age, Theodore was worse than Fowler, right? Fowler has been the better player at every step in their development. Which is why I go back to Theodore's developmental curve needing to steepen, for that to happen.
I'd be very surprised if Theodore surpassed Fowler. From what I've seen, Theodore could be the better offensive player, but it's unlikely he has the two-way capability that Cam has. He also shares some of the same weaknesses, in that he doesn't really engage much physically, and he'll rely a lot on positional play, but he isn't as good on his skates, he isn't as mobile, and I'd say they have comparable hockey IQ. Shea kind of looks like a Fowler-lite at this point.
I just assumed your scenario had Fowler being traded, because that's the realistic scenario right now where Theodore is our #2.
Vatanen is another matter, and I see that as a bit closer. In that case, I actually think Theodore could have the edge in terms of two-way play, and offensively I could go either way. I'd lean towards Vatanen, because we've seen what he can do at the NHL level. Theodore has the height and reach, but Vatanen has that fearlessness. I actually, ultimately, see them competing for the same role, and one will likely mean the other is moved.
I think you need to consider how impressive both Fowler and Vatanen were, at the same age as Theodore. And also how many other very impressive young defensemen end up falling short of top pairing status.