RE: Vent O-ring
When it comes to vent plugs, I never open them unless I'm traveling over mountainous terrain or transporting my boards by airplane. I've never had a problem, even with boards I've been using for 6-8 years. In my opinion, opening and closing the vent just invites unnecessary wear, or even a potential disaster because of forgetfulness.
As Roger has emphasized, make sure there is no water in the board, and then install a new O'ring. I would advise against using any added sealent, because it's really not needed, and it eliminates any residue issues in the event you have to open the vent for some practical reason.