Vanilla Ice -- I Can Save Justin Bieber From Being A Douche

Vanilla Ice fully admits he used to be king of the douches, but says he knows the secret to saving Justin Bieber from the same fate.

Ice -- aka Rob Van Winkle -- came on TMZ Live to talk about Miley Cyrus saying she warned Justin about being so douchey he'd become a joke, a la Vanilla Ice.

Rob doesn't disagree with her, and even shared the formula he used to turn his life around.

Listen up, Justin ... Prof. Ice is talking.