Messaging the guy who did this this. This is what he said: "On renderstep set the viewport frame part’s CFrame to the original part’s CFrame. Original part should be neon, viewport frame part should just be whatever you want shown." – tyridge77
Yep, this made it work fine. Can’t believe I didn’t see that before.
The offset is coming from the GuiInset. Without it, looks beautiful.
When it comes to this kind of camera stuff, I always forget that the blasted GuiInset makes everything ugly. Same with camera functions which is why ViewportPointToRay is advised over ScreenPointToRay for constructing unit rays from the camera.