This is not the vent. This is a dirty dome. I'm almost 100% sure, because I have the same problem with cameras of few of my customers. Right after the installation the picture was perfect, it got bad after some time, when the dust collected on the dome.
The problem is, that I do not know how to clean it. The regular use of windex and soft tissue does not do any good, because the particles of the dust make a tiny scratches on the plexiglass when I try to wipe them away and that is enough for the IR light to diffract on them and bounce back to the camera.
After wiping, the dome is practically not usable for night vision. I really do not know what is the solution to this problem.