I always put my viewports on a noplot layer. You can put it on defpoints (not recomended) or even better just put it on a layer and select noplot in the layer manager. also freezeing the layer will get rid of the viewport border but still leave the viewport.

Now if you want the viewport to be visible this isn't a solution. As a workaround you could draw a border over the viewport on a visible layer that should react to draw order.

The simplest solution may be this however: have you checked merge control on your plot settings? Go into the Plot Dialogue Box. Properties. expand Graphics. Change Merge Control to Lines Merge. (The default is Lines overwrite)

-Steve G.