The Vector is a tiny bit faster than the 840 pro.
That being said, there's no point in buying either of those - the difference between a Vertex 4 or 830 and a 840pro or vector isn't going to be anywhere close to noticeable by a human.
Ummmm no. According to Anandtech the Vector is faster than the 840 Pro in all of 3 of 17 benchmarks. The 840 Pro is generally at the very top of every single benchmark.