Vx, Vy, and Vz are the speed and direction of your entity in meters per second on each axis of the world.
Let's say your entity is moving at 10 meters per second.
If the current frame only takes 1/10th of a second to render, then you only want to move the entity one meter that frame. That way, over the ovurse of a second the entity will have moved 10 meters.
If you did not scale the speed by the time a frame takes in seconds, then you would move the entity the same distance every frame, and the entity would move faster when you have a higher framerate, and slower when you have a lower framerate.
Let's say your entity is moving at 10 meters per second.
If the current frame only takes 1/10th of a second to render, then you only want to move the entity one meter that frame. That way, over the ovurse of a second the entity will have moved 10 meters.
If you did not scale the speed by the time a frame takes in seconds, then you would move the entity the same distance every frame, and the entity would move faster when you have a higher framerate, and slower when you have a lower framerate.