Hmmmm, ok I think you might be a little confused about things. An air fuel ratio will be good to let you know if you're running lean, and a vacume gauge is from what I can tell, pointless, its just something to look at, your engine doesnt require a vacume gauge, it doesnt monitor anything specificly to the point where it would show a problem with anything, unlike a boost gauge on a turbo'd car where the gauge would show if your turbos are spinning to fast and making too much + boost.
So you can keep the vacum gauge with or without Nos installed, if you want something pretty to look at. An air/fuel ratio might still be usefull, but again I doubt its an absolute requirement.