The supermodel, who turned 42 on Tuesday, had to hold up her slipping gold gown at the party with Zorro star Antonio Banderas, a newspaper reported.
She posed on the catwalk in two gowns to raise money for people suffering from HIV and Aids.
Birthday special: 21 actresses who married businessmen
Photos: Alia Bhatt, Sidharth Malhotra watch 'Ae Dil Hai Mushkil'
Spotted: Aditya Roy Kapur with his mother in Bandra
In pictures: Bollywood celebs who share birthdays
'Kahaani 2' trailer launch photos: Vidya Balan takes an auto ride
B-town heaps praise on Ranbir and Anushka
Bollynews Fatafat: Salman's bodyguard in trouble!
'Kaabil' trailer looks promising!
Why was the police looking for Vidya?