Looking at your photo it looks as though the lid vent is slightly raised on the side away from the handle. That is usually caused by the lid being dropped on the handle, causing it to have a slight depression in it. That may be what is causing the lid vent to be loose. Before drilling the rivet out, I would try giving the inside of the lid a couple of whacks with a rubber mallet (right on the two lid handle weld points). That usually pops the lid back into shape and in turn may tighten the lid vent. If that doesn't fix the problem or you don't feel like whacking your lid, I believe the hardware that Weber sends for attaching lid vents are 1/4 20 bolts and nuts.