Form your photos, you do have something on the IR filter (there is an piece of glass in front of the photo sensor that is an IR filter). In the D70 menu there is a mirror lock-up. Take off the lense an use lense brush to brush of the dust after locking up the mirror. The user manual tells you how to do this.

The vignetting, does the vignetting only happens at the wide end (18mm) or through out the zoom? You said that you don't use the lense hood, out door in bright sun it is needed, the 18-70mm lense can give you some very bad lense flare. I had to stop and retake one photo of a mail box because of the lense flare! Yes I have notice some light drop off on the corners of my 18-70mm lense but not anywere as bad as your examples!