veto vs vote down

veto is a noun but vote down is not a noun.

Word	Noun	Adjective	Verb	Adverb
veto	Yes	No	Yes	No
vote down	No	No	Yes	No

As verbs, vote down is a hyponym of veto; that is, vote down is a word with a more specific, narrower meaning than veto:

Other hyponyms of veto include defeat, kill, shoot down, vote out.

veto (noun)	vote down (noun)
a vote that blocks a decision
the power or right to prohibit or reject a proposed or intended act (especially the power of a chief executive to reject a bill passed by the legislature)

veto (verb)	vote down (verb)
command against	thwart the passage of
vote against; refuse to endorse; refuse to assent	vote against