I don't believe you can in OS X - even though it's vector drawn, a lot of assets are raster.

Apple pushed (weakly) a "Resolution Independent" interface in 10.4 (I think) that would have promised drawing the screen at whatever DPI you want without looking terrible (and giving people who want larger text a better option than decreasing resolution)

It didn't really happen; though, and recently I remember seeing them push pixel-doubled assets, ostensibly in preparation for desktop retina displays in the future.

I didn't know GTK3 was vector drawn - that's really awesome!