Its recomended to have some calcium in a vinyl pool for a couple reasons. Water that is too soft can be corrosive to metal. This could have an adverse effect on metal that comes into contact with the pool water such as ladders, rails, and most importantly heat exchangers in pool heaters. Most manufacturers have a hardness requirement for the warrenty.
Now, all that being said, I've worked on copper piping in 40+ yo houses where I know the water was pretty soft (< 50 ppm) and I've yet to see real problems with corrosion that I would have attributed to soft water. I've also heard that elements that make up the vinyl could start to leach out if the hardness level was too low. But, I have seen no real proof or study that this is true, so take that with a grain of salt.
Bottom line if you have a heater, run 100 ppm or so. I keep mine around 100 ppm. Thats enough, IMO, to negate any negative effects on my heat exchanger, but low enough not to worry about scaling and coating my SWG with calcium. Even if I did not have a heater, I would still keep ~100 ppm.