I think you want to do a set -o multiline.
Excerpt from man ksh:
multiline:
The built-in editors will use multiple lines on the screen for lines that are longer than the width of the screen.
BTW, if using the standard vi editor in OpenBSD, not vim, you will get line wrapping at 72 characters with the following in a ~/.exrc :>
set wrapmargin=8
set autoindent
set shiftwidth=4
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