[alst]

I use Eventscripts to solve this.

https://www.mousedown.net/software/Downloads.html

and this script on fast user switch.

-- To receive the eventArgs parameter
-- we wrap the script in an on run handler

-- eventArgs is a variable which will be populated when the script is executed
-- by EventScripts. If you run the script outside of EventScripts (eg in an editor)
-- you'll get an error since "trigger" has not been set in "eventArgs".


on run eventArgs
	
	-- every eventArgs record has a trigger property
	-- that describes the action that caused the script to be executed
	
	set thisTrigger to (trigger of eventArgs)
	
	if thisTrigger is "Fast user switched in" then
		
		-- do whatever you want to happen when
		-- the screen is unlocked here
		
		-- YOUR SCRIPT HERE
		tell application "/Applications/DisplayLink Manager.app"
			activate
		end tell