I have a Vortex laptop. 2 screws of the battery - undone, 12 screws of the panel - undone. It just seem to have something in the middle that wouldn't let it go.
I do not have a vortex but another of my laptops, also with a Clevo chassis, had a recessed screw in the middle, this was hardly noticeable as the screw was quite deep, so worth having a good look.
Otherwise, the Vortex is based on the Clevo PB50RF chassis, an internet search of that may well find some reviews or videos about maintenance.