The difference between the VX-L and the VX-3L is the same as the difference between a VX 111 and a VX-3.
Namely, Diamondcoat lens coatings on the VX-3L instead of MC-4 coating, and the new erector spring assembly in the VX-3L. It's a double bias spring that has higher pressure, and is often referred to as the Mark 4 erector assembly. Whether that is actual/factual technically exactly true or not I have no idea.
Or, you could call them the old model and the newest model. They don't make the VX-L anymore.