From what I read (and I admit I started to skim, part way down page 2) I think everyone's (well, some people are) talking at cross-purposes.
If you have an image (especially a low-res one, like something that's tile-sized, like wot DF uses, guvnor), you can't easily convert that to a vector-description. A small circle might indeed be a small circle, or a small octagon, when it comes down to that scale, and you don't want your small circle vectorising so that it appears like an octagon at larger scales, and nor do you want you small octagon vectorising so that it looks like a circle at large scales.
Whatever's done, it'll need to be done from scratch. (Or at least with a lot of manual help to guide the automated edge-detection routine to the (possibly even then, subjectively) right conclusion.)
Ultimately, I could see DF going vectorised, but I don't see it being done within the paradigm of the current interface. I'm not entirely sure why people zoom in to DF. Pixel-for-pixel, the standard rendering of the (standard[1], or even the 'squarified' version theoreof) tileset gives enough information to (when you know what they are) identify everything for what it is, and distinct even from all similar-but-not-identical tiles.
Right now, I think the situation with raster graphics (that may or may not have been originally generated through rasterising a 'base' vector graphic, and touched up as necessary in the smaller examples) produce for each size would be the answer. Perhaps an option in the tileset specifications that mean once a tileset has been magnified so that it's more than 2x2 pixels (displayed) per 1x1 pixels (on the tileset reference) it switches from Tileset1 to Tileset2, and then when it goes beyond the next limit (4x4 original pixels, 2x2 pixels since the previous switch) it adopts Tileset3... or something. Inbetweentimes, it'd have to do something like cubic approximation to translate N source pixels over a stretch of M display pixels (where neither N/M or M/N are easy integers).
Note that in the above example I assumed it's making it 1 pixel bigger per pixel (per axis), but I think (from the last time I checked the INITs) that it actually adds 10 pixels per zoom level, or something (regardless of how many it already zoomed, or does it go proportionally larger, thus exponentially?), but the same principle applies. Anyway, you'd still only be guaranteed a 'nice looking' sprite when it coincides directly with a size given in the set series of icons, and perhaps decent looking ones when it's composed of an integer multiple of the most immediate smaller size (or, if you're lucky, if it's an integer division of the immediate larger and that it's actually reducing), but either aliasing effects (for expanded bitmaps) or blurring together (for contracted ones) could still exist even then, and would definitely cause some artefacts at all other intermediate positions.
Sorry, meant to keep this brief, and seem to have spontaneously come up with my own 'solution'. Although I'm not particularl championing even this idea, just putting it forward as "best of both worlds", for those that care for such things. (With apologies if I missed anyone saying anything similar, and less wordily.)
[1] Not wanting to make this a default vs. 3rd-part tileset debate, so sticking to the basics.