It would be really helpful if you could go into more detail on your use case and the problems you are trying to solve. (see How to post a Feature Request).
From what you have described, you could only add VecorForces to Parts where (Part.AssemblyRootPart == Part), which should guarantee only 1 VectorForce per assembly.
It’s important to note that there is no “assembly instance” that could accessed in the way you are proposing. It’s simply a set of connected parts. Have you seen the latest physics API additions here?