I know you can use polygon viewport, but is there a way to get a smaller viewport to "float" above a larger one behind it so you don't see the larger viewport behind the smaller one?
You will have to use a wipeout or a hatch with 0% screening behind the smaller viewport to hide the objects from the bigger viewport. You could try to "cut-out" the portion of the bigger area by using a polygonal viewport by playing with the plines and overlapping some plines.
You can also create a hole in the larger viewport. not obvious but it can be done.
1. Draw the out line of you viewport and the hole as closed Polylines
2. convert these to regions
3. Subtract the smaller region from the larger
4. convert this single object to a viewport
5 create you smaller viewport in the hole of the first (you may want to leave a little blank border.
Do plan carefully because as far as I am aware you cannot edit the shape of the viewort you created from the region