I don't know if a diffuser might help, but without one I have had a charming butterfly's silhouette on a cyanotype, and a cat also managed to ruin another one by sitting on the same chair against whose back my printing frame was resting...
On a more constructive note, if the edge of your VDB image is too close to the the frame of your printing frame, depending on the time of the day you are exposing it, it might be affected by the shadow cast by the frame. If it is a short exposure you can check and alter its position, but if it is a longer exposure and you are not going to stand and watch over it a diffuser might help.