So, it seems, Blender has got vector icons engine recently?
Toolbar icons (blender installation folder …/datafiles/icons/*.dat), are binary files that contain the coordinates of the vertices of the polygons and the color of each vertex. You can play with this files in hex editor.
So it is more a 3D file than a vector file. There are no smooth Bezier curves and you can see it’s low poly if you zoom in.
Icon format and source file:
https://developer.blender.org/diffusion/B/browse/master/release/datafiles/blender_icons_geom.py
https://developer.blender.org/diffusion/BL/change/trunk/lib/resources/icon_geom.blend
UPD: This discussion can be moved to the About Blender Icons, since it’s off-topic.