Followup: Delta leases Terminal 2 in its entirety, therefore they get to set the rules. Vaping is allowed. Terminal 4 is not just Delta, therefore the Port Authority sets the rules. Vaping is not allowed. You can't just go back and forth between the two terminals at will - you need a boarding pass for Terminal 2 to go there. So if you've just come into Terminal 2 with a connecting flight in Terminal 4, once you leave 2 you can't get back.