(vec coll) and (into  coll) do exactly the same thing. However, due to into using transients, it is substantially faster. On my machine:
(time (dotimes [_ 100] (vec (range 100000))))
"Elapsed time: 732.56 msecs"
(time (dotimes [_ 100] (into  (range 100000))))
"Elapsed time: 491.411 msecs"
This is consistently repeatable.
Since vec's sole purpose is to transform collections into vectors, it should do so at the maximum speed available.