From looking at the questions of each I couldn't quite decide exactly what semantic field they cover and how much they overlap, but when asking some questions I have myself been stumped as to what the best tags would be, even including the possibility of creating new tags; and I have gone with vocabulary as one of my tags.
Yet I'm still fuzzy on describing what sort of question need that tag!
How do the rest of you feel? What areas should this pair cover? Should they be merged? Would somebody like to take a shot at creating the tag wiki for vocabulary at least?