I can't find a link to it, but I remember reading a discussion, that if you want icons/images for fixes known sizes and resolutions, it is better to have rasters than vectors: I think, mostly to do with scaling of line weights/widths, vector scaling of these often results in ungainly images while a raster of a given native resolution looks clearer/better more proportionate.
To the extent that might have ever been true, it would have been because of the limitations of very low resolution displays, or to work around limitations in the vector rasterizer.
In any modern context, a vector is always preferable as it will generate the best output for an unlimited number of sizes, resolutions and contexts.
The question may have been the scaling of line widths in a linear fashion while using vector images, which isn't that pleasing to the eye, compared to a hand crafted raster with more eye pleasing line width for a give size/resolution. May for aesthetic reasons line widths may require to be scaling in a nonlinear way?
Scaled vectors will have mathematically correct line widths, but to a human perception the line may appear to thick/thin for the image size, than which a raster artist would have used if they were creating a native raster of that size.
