Saints' win puts them above the Cherries into fourth place and firmly in the automatic promotion shake-up.
The home side opened the scoring after five minutes when Radhi Jaidi blasted the ball against fellow Saints defender Jose Fonte. The ball fell perfectly for on-loan forward Lauri Dalla Valle who fired a half-volley past goalkeeper Kelvin Davis.
Southampton levelled six minutes later when Rhoys Wiggins was caught on the ball by starlet Alex Chamberlain and he crossed for the in-form Lee Barnard to stroke the ball home.
The match was heading for a draw until Chamberlain crossed for captain Dean Hammond with 20 minutes to go and he slotted the ball back across Shwan Jalal and into the corner.
Lambert made the game safe three minutes from time when he smashed a 25-yard free-kick past Jalal to put Saints firmly in the driving seat. Oscar Gobern was sent off in the dying moments for a two-footed challenge on Shaun Cooper.