I think to replace the current vector tools would not make much sense though I think it would be more of an improvement of the current in terms of usability than changing the core of it. Krita does alot of vector work already it is just hard to use I think like drawing it but it is good.
I also did a proposal just like this and I also had greese pencil in mind. I tried to tried to make sense for Krita’s structure and working in a 2D plane first before thinking out of the box but a small basis would allow more growth even it was a plugin working over it later. Also I wanted to draw with just vector as it would be much more clean.
As for using blender code as a module I am not sure if that is directly applicable sounds too good to be true with Qt I get a feeling it would fight against it placed outright.
The thing is Krita would not need as much 3d stuff even if was implemented and greese pencil has to consider tons of other things I imagine. Probably something inspired by would make more sense and would not cause maintenance issues.
But a bridge with blender would be dope. Krita feels so much more natural to draw but I think that would be better for texture painting more than animation as a best use case first. But with Qt6 it might not be needed if it ever gets implemented.