Index shifting. The actual index is in the shifter. The cable housing needs to be linear. The rear derailleur is designed to function in the system. With Shimano, the guide pulley has some side to side play. The gear cluster has optimized ramp patterns and the teeth are offset to make for better shifting. All the parts work together for the index shifting. It may work with one element missing, but not optimally.

The Suntour Vx isn't designed for an index system. It may work, but since it isn't designed for it, I believe it will be a problem. A derailleur designed for indexing will work with a friction shifter, but not so much the other way around.