I don't know why, my post has been edited and the most important part of the question is missing..
The question about updating the main model via a ViewModel that has "some" of the properties of the main model.. is it correct to do it this way?
public ActionResult Edit(modelnameViewModel viewmodel)
modelname model = db.modelname.find(id);
return PartialView("FullDetails", model);
This actually works, but I'm not shure about it being "formally correct" or not