(1) The zone you enter or feeling you get when the alcohol starts kicking in. When you have the confidence to walk with a bounce but maintain ability to do so. The point where your 'dance' and 'game' levels reach equilibrium. And you can have fun while maintaining your swag. Beyond tipsy but before drunk.
(2) It may also refer to a couple flirting or a bond/connection between them. Or when a man is making successful moves on a lady.
(1) "I was vibing when I walked into the club" "let's get vibing"
(2) "John and Jane were vibing" "I'm vibing with her"