Vanessa Hudgens puts the blame for porking up 15 pounds on her painful breakup with "High School Musical" co-star/loverboy Zac Efron - and producers of her new flick "The Frozen Ground" have hired a personal trainer to whip her into shape fast for an October startup. Vanessa - who'll play a teenage victim who teams up with State Trooper Nicolas Cage to catch serial predator John Cusack - just "let herself go" after her hot romance with Zac went cold, said a pal. "She wasn't working out, and was eating tons of junk food!" Vanessa's fitness guru workers her hard seven days a week, she's on a restricted calorie diet, and she's lost about eight pounds already. (Print Edition - 10/17)