Well, as It was cleverly proposed in another thread, the vertex ID number changing with subdivided meshes (depending from the sub-D level), using the UV coordinate would be a better and more stable option to have a mesh / null stick to a certain part of another deforming object.
I updated the DPkit to the new 2018 version (thank you so much Denis) but I can't find a node network that would allow me to do that.
Any body has a clue?

Thank you.