At risk of sounding like the guy who throws the machine learning hammer at everything, I actually wonder if a tiny CNN would not be a great solution to this. I

At risk of sounding like the guy who throws the machine learning hammer at everything, I actually wonder if a tiny CNN would not be a great solution to this. If you have a bunch of example icons tuned for low resolutions, as well as the vector art, you could potentially train a network to do a better downscaling job specifically tailored for icons. The point would be that the CNN weights could be common to all icons and not necessary to store in each file. Since the "domain" of icon design is quite restricted in terms of dimensionality and colour space, I bet a pretty small CNN would do a great job.