I don't think the VRAM indicator in GTAV is even remotely accurate.
Yeah mine is miles off also, i wouldnt worry bout it . Apparently im using 6500mb/6100mb and after burner says im only using 5000mb.
As long as it runs smooth and looks nice its all good.
The game is so undemanding ive acatully dropped my cards down to 800/1250 which is my energy saving profile i use in wow, cause im getting cooked in my room here my pc is kicking out so much heat. As long as it stays around 60 fps i dont mind.