I want to add a short mpeg (credits) file to a finished mpeg file. Do I just re produce them on timeline and will this reduce quality on first mpeg file?
Yes. It will reduce quality, because it's going to re-encode. I don't think vms supports no-reencoding for mpeg2, only for DV apparently (and vegas pro can do hdv too), but i haven't heard of the feature existing for mpeg2.
Vegas Pro 8a can perform 'no re-encoding' editing of general MPEG-2 files, but it's not an official feature (probably because it's tricky to set up the MPEG render settings so that it works). Hopefully Sony will improve this in future versions.