I've refreshed my memory of this and can see why it's confusing, I think it's just wrong.
Shortest vector applies a _high_ pass filter to the vector art, so only lines that are longer than this value (measured in motor steps) are drawn. This was really designed to remove noise - small lines which are almost coincidental. If the short line is isolated on it's own, then it simply disappears. If the short line is in the middle of a long line, then it'll tend to get collapsed, and you'll end up with a long line left over.
The angles of the lines have no bearing on this filtering: only the length. It'd probably be a good idea to preview the effect, but right now you need to do a "draw vector" and see what gets built.
So that feature is related to cycle polygonizer and polygonizer length in practice. If you use polygonizer:1 with a short polygonizer length, then even your long straight lines get chopped up into short lines. Which makes them liable to be discarded by a low "shortest vector" setting. Can lead to some odd interactions.
The other setting is "max line segment length", there's no way to preview this except to draw it on paper, so I'll do an illustration.
Imagine you want to draw a horizontal line like this:
The blue lines here are the grid of the natural axes of the Polargraph, so you can see the horizontal line is actually a kind of distorted diagonal as far as the machine is concerned. It can only really move a) directly along one of those blue lines (only one motor moving) or b) move two motors at once and move diagonally. Because of the odd nature of the coordinates system the diagonal still doesn't result in a straight line on the page, it's kind of hooked.
So to approximate a straight line on the page, the line is chopped up, and while each smaller line is a bit hooked, over all the line looks straight.
Max segment length controls the length of the segments. A long setting might chop this horizontal line into only six segments, and look like this which has significant and noticeable deviations from the intended path:
while a short setting will mean the deviations are tiny, and possibly unnoticeable, once you take the natural hysteresis of the pen tip into account:
Smaller setting are slower and noisier. You might use larger settings if you were drawing something very large that wasn't going to be looked at up close - or it was just an effect you likes. If you have noisy random looking images anyway, you might not notice it. On the other hand, there's a good chance that you already have lots of small lines on that, so they wouldn't tend to get chopped up anyway. If the straight line is smaller than your max segment length, then there's no chopping.
Hope that makes sense!