I need to put the top vent back together electric fry pan

Fix #1
These are usually riveted on. If you can find a small enough aluminum pop rivet (most lids are aluminum), you could pin it back together with that. The tricky part is ensuring that the rivet allows the vent to rotate between the opened and closed positions. I suggest cutting out two pieces of non-corrugated cardboard from a cereal box and punching out a hole in each the same diameter as the rivet. Use one on each side of the lid for a washer for the rivet. Include an aluminum washer for the tail end of the rivet. Your parts stack from the inside of the lid should be rivet (head end), cardboard washer, vent piece, lid, cardboard washer, metal washer. The pop rivet will be compressed against the metal washer on the outside when you operate the riveting tool. I suggest having the head on the inside of the lid because it will be easier to clean.

After you compress the rivet, tear the cardboard washers out. The vent should now pivot correctly.

Fix # 2
Use a 2-56 x 1/4 stainless steel screw (or similar metric size) and two stainless steel nuts. Assemble the vent to the lid using the screw and one of the nuts. Tighten the nut only until it starts to drag on the vent, then back it off slightly. Next, put the other nut on. Hold the first nut in place and tighten the second nut down onto it to lock it in place on the screw. The vent should still turn freely, but the nuts won't come off without tools if you did it properly.

Do not use plated or plain steel hardware. These will contaminate your food.