Open the mixer view in Nuendo and locate the audio track on which your vocal part appears. Double click on the label at the bottom of the mixer strip.
Double click in one of the insert spaces above the volume fader and select "Noise Gate" from the list of available plug-ins. Double click the icon in the mixer strip to open the interface. Turn the threshold control all the way down to zero, so all the vocal is cut out, then gradually increase it until everything but the background noise is audible. This will give a good, clean starting point for the other processing effects to work on.
Double click another insert slot on the channel and select an equalizer. This will remove all the frequencies not being used by the vocal part, which could be harboring more unwanted noise. Create a high-pass filter, which will chop off all the very low bass frequencies not likely to be used by a human voice. Turn up the frequency control until you hear the vocal start to sound tinny, then roll it back down so it's just on the edge of the vocal, not affecting it directly. Repeat the process with a low-pass filter.
Double click a third slot in the channel strip inserts and select a compressor. Compressors are subtle and there's no right or wrong way to set them up. They should make the volume changes in the vocal level out, without making it sound dull and lifeless. Turn the threshold control down from 100 percent until the activity meter starts to show about 50 percent gain reduction. Then adjust the ratio control until you get the desired sound. Compressors are complex effects, and you may need to continue fine-tuning the controls until you achieve the desired effect.