During drag and drop and other movement, if the appropriate trait is present…
The second item to make VASSAL more usable for miniatures would be collision detection. That can be a can of worms - I have tried it myself in Visual C# and it was not too bad but I do not know if Java has the same kind of pre-canned graphical object methods or you would have to write your own. The idea is when a unit is being moved (alone or part of a group) and is dragged over another unit the collision detector freezes the move and prevents the units from overlapping. This would obviously have to be a trait that you could put in the units that need it and not in others.
Collision detector ‘freezes’ the move is not the best way to describe it, just think of it as imposing a restriction from overlapping so you can drag up to but not past that point, but you can continue dragging back again etc as long as you do not overlap. Make sense?