In speed mode, the autothrust varies engine thrust to maintain the pilot selected or the FMGC (Flight management guidance computer) calculated speed. In V/S mode, the priority is the selected vertical speed. If the vertical speed is high to the point, the set speed cannot be maintained with the set thrust, the aircraft will decelerate or accelerate depending on the circumstances.
In the Climb phase:
When we pull the VS mode for climb, thrust goes in Speed, but what is the thrust value?
That depends. If you are say maintaining 290 knots and you pull for V/S, vertical speed will go to whatever vertical speed that maintains 290 knots. At this point, if you increase your climb rate, then the aircraft will pitch up to maintain the target V/S. Thrust will also be added to maintain 290 knots. But once the thrust is limited (in a climb, max thrust is climb thrust), the V/S will be maintained at the expense of speed. i.e. the aircraft can no longer maintain 290 knots.
In the descent phase:
When we pull the VS mode for descent, thrust goes in Speed again, What if I want to maintain ROD 1500/min and reduce speed from 280kts to 250 kts at the same time? Will the Auto pilot guidance be able to do that?
In this situation, if you pull for a V/S of 1500 ft/ min, the aircraft will pitch down to maintain 1500 ft/ min. So, naturally, the thrust will go towards idle. Now, if you reduce speed, the thrust will go down again maybe to full idle in order to reduce speed. If the aircraft is at a V/S 1500 ft/ min when the speed is reduced and if the speed goes below 250 knots, then thrust will be added until 250 knots is maintained. If the aircraft is unable to maintain 250 knots with a V/S of 1500 ft/ min at idle thrust, then the thrust will remain at idle but the speed will increase to a point where it maintains 1500 ft/ min descent rate, again at the expense of speed.