A vote for trump is a vote for trump.
A vote for clinton is a vote for clinton.
A vote for johnson is a vote for johnson.
And a vote for stein is a vote for stein.
If you want to vote "Not Trump" then there's three other choices.
Same with "Not Clinton"... three other choices.
That doesn't include write-in option nor even smaller parties than the four.
It doesn't say much for a candidate if people are only voting for them as a symbol of "not-other-candidate".
Nor does it say much if a candidate can't get enough support based on their own values/principles/actions.
Imo, to say things like "a vote for [anyone other than Clinton] is a vote for Trump" is actually putting Clinton down as well...as if she has nothing else to offer other than being "not Trump".
It's also a cheap scare tactic used to try to coerce people into supporting establishment politics instead of fighting for political change. It works...because people in the past kept falling for it, allowing the establishment politics to get further and further entrenched and harder and harder to change.
On a personal note, I'm sad Sanders didn't get closer than he had.