Honestly it isn't his age that concerns me.
If he truly is just a late bloomer coming into his own and has miles left, then it would be good. Full credit to him the last couple years, he's earned his shot.
I'm just skeptical about him long term. Keep him hungry, and if it falls off you don't have to worry. I really don't see a reason to commit long term (one could argue 3 years isn't long term I guess) to a bottom pairing defenseman. They did the same thing to Kulak because he was playing in the top 4. But when you start acquiring players for roles they're actually suited to, he gets bumped down to where he should be, and then costs way more than the position should.
Just my opinion, of course. I see VD as a 6/7 tops, so we may disagree there.