A slight vac leak will still let it stay steady(usually just a bit lower of a reading) as there's no intermittent interruption in the intake tract(which would make the vac gauge jump around) only that the mixture air/fuel ratio in the intake tract is now off enough to cause driveability issues. but if it got lean enough to cause a lean misfire then that would cause the gauge to dive for a split second as that cyl didn't do it's job.