The real answer here is nobody knows, and it will likely be up to SCOTUS. Everyone saying it needs a constitutional amendment doesn't understand what NPVIC is. The whole point of it is to do an end-run around the constitutional amendment process - a hack if you will.

Whether this hack is legal is a complicated question (https://en.wikipedia.org/wiki/Constitutionality_of_the_Natio...) that realistically can't be answered by anyone but SCOTUS. If states pull the trigger on NPVIC then I would bet good money that it ends up in SCOTUS, but who knows how they will rule.