Excellent Ali. You're moving forward faster than me.
The vacuum sensor will connect straight to the servo on the standard servo connection. This will then be where Audi intended it to be, making the plumbing pretty easy to achieve. In reality it won't make a whole lot of difference where it goes, but why mess with making home made pipe connections, if Audi have already done a majority of the work for you. The sensor itself can be mounted on a simple bracket in a nearby location, giving an OE look.
I'd actually mount the vacuum pump to the Leaf motor on some form of bracket, as the mass of the Leaf motor, will damp out most of the noise from the vacuum pump.
Don't mount it on a part of the body if you can avoid it, as the body will act like a sound board, and amplify the noise.