if performed properly, the performance/ vibration/ smooth power issue will not exist as an issue.
On one of my engines I did this to, it was a v-8. so every OTHER cylinder in the firing order was disabled, and the system was used like this, and the engine actually sounded no different than normal, and there was no shaking or roughness to the power (like serious misfiring).
on v-6 engines...a little tricky. inline 5, almost impossible, and certainly impossible on a straight 3. 2 cylinder, 4 cylinders, inline 6, and v-8/v-10/v-12 engines work well.
For lack of a good "Unused" vacuum port, put a "T" fitting in the LARGEST one, and just use that, but make CERTAIN that it is NOT the BRAKE BOOSTER LINE!!!
AlaskaStar