My AC vents change positions rather easily which changes where the air blows. >
> I am looking for ideas on how to stop it.
>
> I thought about using vaseline in the circle area, but my cars gets to
> 140 degrees in the sun.
>
>formatting link
If they move real easy they are worn or poorly made. What you can do it to pop the vent out of the hole then look at the link that secures the vanes on the back. There is a ring that holds the vanes into the directional head, they can get loose. Once the vent is out you can usually add something like an oring or rubber band to one side to make the vanes a bit harder to move.