Lambert showed a cool head and took his personal tally for the season to five goals and give Saints boss Nigel Adkins his second win since taking charge.
It might have a different story when Warren Feeney struck the post in the opening minute for the Cherries but that was as close as the visitors came to scoring. Southampton dominated possession and territory and the surprise was that they only led at the interval by one goal.
Bournemouth keeper Shwan Jalal had already denied Lee Barnard when Lambert was sent in with a chance to fire Saints ahead when Rhoys Wiggins handled.
Lambert extended Saints' lead five minutes into the second half when Marvin Bartley shoved Morgan Schneiderlin in the back to present Lambert with his second penalty, which he converted.
Bournemouth never seriously threatened the Southampton goal in the second half and their misery was complete when defender Adam Smith was sent off five minutes from time for a second bookable offence.