I did some search on google and I found someone asking a similar question on another forum. This was the answer he has got:
On some nVidia GPUs it seems only the driver installing process enables the GPU VRM sensors, so if you install a new driver, you might see them in HWiNFO.
Unfortunately there's no solution how to make them visible always, since nVidia won't tell how to enable them, nor doesn't leave them visible all the time.
Just checking if anybody has any other thought on this and if there is any way to enable VRM temps in the NVidia drivers, without having to reinstall drivers all the time (if the above is true). Anybody?