Quote (from I Fly Indigo)
"sorry but it sounds very stupid to me
... What I learnt and noticed is that once you select V/S mode, selected vertical speed governs the engine thrust.."
OOOPPPSSS, I hope that is a shortcut only in writing, not in understanding, because what does your FMA tell you when you select V/S?
It says SPEED - V/S
This means that AutoThrust controls speed and Pitch controls Vertical Speed.
Now, depending on the Vertical Speed that you select, thrust will be anywhere between idle (usually for a descent) and climb thrust (for a 'good' rate of climb). If you select a very high rate of descent, the speed will increase above command speed, even at idle thrust. If you select a very high rate of climb, the speed will drop below command speed, even at climb thrust. That is why V/S is not a fire and forget mode, as pilot you have to stay closely in the loop.
As far as fuel economy is concerned, like others stated already - max (i.e. CLB) thrust, at best speed to best level and than cruise at best speed to top of descent, which should bring you to short final at best speed all the way in idle thrust. Just dreaming, doesn't happen often in the real world.