I run the one master power brake sells, I went that direction because it has a pressure switch that cuts the pump off when vacuum is achieved, that also applies with your foot holding brake.
My pump is located under the car and I have side pipes that are loud so I only here the pump before I fire the car. I messed with the noise for grins just to see if I could knock it down some.
Here is what I found that little muffler is about as quiet as it gets, tried to run a rubber tube and attach the muffler at the end , no real difference in db level. about the best way I figure to get it down might be to completely enclosed it like you would do to a computer. You could trunk mount it that way.
Something else I tried was a reservoir tank to see if I could cut down the cycle time, and all I learned there was that it made the pump run longer because of the volume increase using a reservoir.
I would be interested to know the pump you end up using, for more knowledge.
Here is a link you may not have thought of RVs use these pumps and I would guess noise is an important feature in that application; http://www.readybrake.com/electricvacuumpump.html
As I said before my car is plenty loud and I do not here the thing once the motor is running.. What's that saying loud pipes hides the vacuum pump noise or was that saves lives ? So there it is all I can think of regarding the use of an electric vacuum pump, if you think of another question let me know...