I'm assuming that they used the Vx with the long GT cage, which also has the split chain feature that allows one to slip the chain from within the pulley cage without breaking it. Yes, Vx is superior to that SIS derailleur, but both would shift acceptably in that set-up (includes your flat cogged SunTour freewheel).
Is that a 30-tooth largest cog? I wonder if the shorter cage Vx could reach that. There is a B-screw adjustment.
Your drivetrain needs cleaning. The chainrings should shine. A little WD40 onto a rag or thick shoelace and go to town. Don't spray onto the bike, because splatter will get on your rim walls inevitably.