Careful - registry involved:
;; DvcNme = Device Name [STR]
(defun PutDefPlotDevice (DvcNme)
(vl-load-com)
;; Lee Mac ~ 14.01.10
(if (vl-position (strcase DvcNme)
(mapcar (function strcase)
(vlax-safearray->list
(vlax-variant-value
(vla-getPlotDeviceNames
(vla-get-ActiveLayout
(vla-get-ActiveDocument
(vlax-get-acad-object))))))))
(vl-registry-write
(strcat "HKEY_CURRENT_USER\\" (vlax-product-key)
"\\Profiles\\" (getvar "CPROFILE") "\\General")
"DefaultConfig" DvcNme)))