If I am missing something I hope someone will point it out, but since the vector video options changed* (with the v0.107 video makeover I believe) the vector output looks poor.
(* translucency and intensity removed)
This is particularly so in low-res modes which before could be tweaked to look very good on a low-res screen. The difference (compared to older versions of Mame) is not quite as obvious in high resolutions, but still not as good.