Horrified by photos of singer JOHN "I Ain't Nothin' But a Hound Dog" MAYER lolling la dolce vita with JENNIFER ANISTON in Miami, funnyman VINCE VAUGHN - still toting a hot torch for his ex-flame - lost his sense of humor and phoned to warn Jen about the legendary ladies' man.

"You can't be serious - this guy is just using you to make a name for himself," Vince told Jen, said a source - then added that he'd heard man-about-town John had actually tipped the press about their dates.

Jennifer, knowing Vince aches for a rerun of their failed romance, was "highly amused by the whole conversation," revealed the source.

"She told Vince that in case he didn't notice, John already had quite a name for himself - with or without dating her - and if Vince didn't like the reports that were surfacing, he should stop reading the papers and spare himself a heart attack!" Yee-OW!

• HEIGL'S MAN HITS A HIGH NOTE
• CHRISTINA AXES PITY PARTY
• TINA TURNS PURSE THEFT INTO $$$
• NICK LACHEY: REAL MEN CRY IN PUBLIC!
• JULIA WANTS THE SCOOP ON ANGELINA
• SIMON'S GAL PAL GETS COZY WITH STEPHEN DORFF
• KATIE'S DRESSING ROOM: NO SCIENTOLO
• RYAN ROLLS WITH THE PUNCHES
• NEW CELEB FEUD: RYAN VS. SHIA