Correct, VST or "Vertical Speed to Target", is the rate you should climb or descend to reach your desired altitude. This instrument only shows up when you need it. Example (presuming no changes to the default settings) the app assumes you will want to arrive at your destination airport's pattern altitude 2nm out. So when you need to begin a 350fpm descent, the instrument shows up reading "350".
If you continue straight and level, the required descent rate to reach pattern altitude will steadialy increase. So if you want to descend at 500fpm simply wait till the number reaches 500, then begin your descent.
Thanks,
Walter