Britney Spears -- I May Not Sing In Vegas, But I CAN SURE DANCE

Britney Spears got the lead out for tonight's big Las Vegas debut.

Brit's friends and family were there for her final dress rehearsal for the "Piece of Me" show ... and she did not disappoint.

Britney is back to her old self onstage ... her dance routines blew everyone away. Her singing ....