Vignetting the print is much easier and you don't ruin a perfectly good picture if you don't like it afterwards... but if you want a negative with vignette, I recommend using a cheap UV-filter (aka protection filter), coloring it with black marker pen and then carefully rubbing away some of the paint in the middle. I had rather nice results with that, though I used a broken dark yellow filter for that Lomo-look. It's not very useful for wide angle lenses, smaller apertures and close focus though, because it looks really ugly when the dirty filter comes into focus.