Vanish is very useful when exploring areas. Want
to pick up a seashell ring? Go into Shadow Grove
and vanish. Nine times out of ten you'll be
halfway to the ring! It's also great for getting
out of no-exit rooms without wasting a recall.
As for using vanish during a fight, whatever the
other guy has to do to mess up a ninja's vanish
(probably trip or bash) I've never been in a
fight where the guy or his pet didn't do
whatever it is that makes vanish useless.
I don't know the odds, but I bet you flee works
more than vanish.