I think the VFI is a general indicator of your rate of fall and a eyeball of the fact that you would take catastrophic damage if you hit the ground... Since glide is taking you down toward the planet at 2.5 km/s fall, it's letting you know if you hit the ground, you would explode. As you slow out of glide, you approach a more safe speed.
Unfortunately that gauge is full of
.In an SRV you can land with it fully red and take no damage, likewise if it is not red you can seemingly just explode. It really isn't useful, especially at altitude in a ship.