Just a thought: Seems like it would be useful for any support for vulgar fractions to make use of the fraction and fraction component glyphs in whatever font is in use.
For cm and ec, there are enough design sizes that using a smaller font for the numerator and denominator is reasonable. MM fonts with optical scales also allow for this. But most fonts don't work nearly as well. Expert fonts however contain all of the required components to produce high quality results. Some even have the correct kern pairs so that just setting eg U+2075 U+2044 U+2087 (to use the unicode character values) would produce a well set five-sevenths. (The adobe ps names for those characters/glyphs would be /fivesuperior /fraction /seveninferior.) (As a side note, Unicode has both U+2044 FRACTION SLASH and U+2215 DIVISION SLASH; Adobe fonts use the same glyph for each character, for compatability with wgl4....) -JimC
