The type of TA/RA issued is really 'time-to-impact' dependant, so relative velocity (closure rate) must be taken into account as well.
Along with relative bearing, altitude, and vertical speed, of course.
There is no such RA as 'REDUCE DESCENT', but there can be an 'INCREASE DESCENT'.
'ADJUST VERTICAL SPEED' is often given in situations where the current a/c vertical speed must be reduced (either in climb or descent) - in fact, 'ADJUST VERTICAL SPEED' always means reduce V/S.