This one will set all viewports shade plot to hidden, it even works with multiple layouts.
(defun c:test (/)
(vl-load-com)
(setq doc (vlax-get-object "AutoCad.Application")
adoc (vla-get-ActiveDocument doc)
lao (vla-get-Layouts adoc)
cnt (vla-get-Count lao)
inc 0
)
(repeat cnt
(setq cvprt (vla-item lao inc)
inc (+ inc 1)
blk (vla-get-Block cvprt)
)
(vlax-for itm blk
(if
(vlax-property-available-p itm 'ShadePlot)
(progn
(vla-put-ShadePlot itm acShadePlotHidden)
(vla-update itm)
)
)
)
)
(princ)
)