Does decreasing volume of some recorded parts affect quality??
 

I want to decrease volume of some fragments I recorded with nuendo and straight into the audio interface. That will decrease sound quality of that parts???

it does not if thats all you do, you know, just lower the volume! but then am not sure if that is what you mean?

Yes, I mean lowering the volume of some phrases that stand out a lot.

as long as you´re not using some crappy device or broken vst, there won´t be any problems.

you can do it with a compressor (decreasing volume at any place that´s louder than the comp´s threshold), or manually via volume automation (=> google).

depending on the song, recorded track, and many more things, i sometimes do more than a hundred automation steps in a song. If done well, it´s certainly an improvement

The original track won't be touched
You're just sending a lower level to the bus for the mix out.
The original will still be there for touch up if you need it.

Using automation is what a DAW is all about. Automate, VST, etc. after the track, so you always have the original.