You could
- set a large minimum width for the line numbers column via
:set numberwidth=6
and - then you could resize your window with
:set columns=86
(or with the mouse) to the proper size.
If you edit a file with a million lines in it, you may have trouble, but that’s unlikely. You’re wasting 6 columns of screen real estate this way too. So there are still all kinds of problems.
You can highlight past the 80th column using :match
like it says here and here.
Beyond that I can’t see any way to do this. Seems like it’d be a nice feature though.