They are all add-ons of one kind or another. I put in the Vapor as I needed to replace my speedo too. There's a bit of a delay as well, and the bar is not as intuitive as a dial. Does the job but not quite as good as a 'real' tach.
It was on sale and cost $25.83 including shipping et. al. They also had a volume discount. If you buy 10 or more it is 40% off. Something to keep in mind if you can find 9 other people that want one. (Or if you can only find 5 other people, go ahead and order 10 and get the discount and have 4 extra for free! The cost of 6 w/o the discount equals the cost of 10 with the discount)