vex vs riddle

vex is not a noun while riddle is a noun.

Word	Noun	Adjective	Verb	Adverb
vex	No	No	Yes	No
riddle	Yes	No	Yes	No

As verbs, riddle is a hyponym of vex; that is, riddle is a word with a more specific, narrower meaning than vex:

Other hyponyms of vex include mix up, stump, elude, escape.

vex (noun)	riddle (noun)
	a coarse sieve (as for gravel)
	a difficult problem

vex (verb)	riddle (verb)
be a mystery or bewildering to	explain a riddle
subject to prolonged examination, discussion, or deliberation	set a difficult problem or riddle
change the arrangement or position of	speak in riddles
cause annoyance in; disturb, especially by minor irritations	pierce with many holes
disturb the peace of mind of; afflict with mental agitation or distress	separate with a riddle, as grain from chaff
	spread or diffuse through