Why the hell would NJ ever do this

Probably because they are currently paying over $17MM on their right defence alone.

With Subban and Severson above Vatanen on the right side depth chart as well as Vatanen's contract expiring at the end of the year....it makes a lot of sense.

Just because New Jersey isn't up against the cap, doesn't mean they're happy spending over $4MM on a third pairing defenseman.

That's why......

But there is absolutely no chance Brandon Manning will ever be an effective player in the NHL again, there is some chance Puljujarvi won't play in the NHL this year, and there is more than a little chance that the third-round pick won't ever be an NHL player ever and certainly won't be this year, so why would New Jersey give up an actual NHL player, no matter how you much you devalue Vatanen, for an "if" and two "no's"? People don't give away roster players who have some value (otherwise, why would you be trading for him?) for nothing.