Yeah Buddy is derived from Yeah Guy. (e.g. sure, whatever you say, not buyin' it). Yeah Buddy on the other-hand is an expression of excitement. Very poistive. (e.g. someone calls, you know your heading out on a Friday night, vibes are good - "Y e a hhhh, Buddy". Here, the Yeah is dragged out some with the "buddy" portion being quick and to-the-point.
Hey, you lookin' to mack some poW up north this weekend-"Yeah buddy".