>> about the difference between ViaCad & MoI3D

Apples and oranges. One is a CAD program (with all the implications and complications), the other is a modelling program.

Make your choice what you want to do, then pick your weapon. Some use Rhino or MoI for modelling and sketching out ideas. And then Solidworks or Autocad or whatever when it comes to actually making things. Not that that can't be done in MoI, but it quickly becomes a pain in the neck because there aren't (real) solutions for standard parts libraries, assemblies, collaborative engineering, parametric design, constraints and dependencies. The list goes on ...