It's a classic Hollywood story line: a lonely outsider follows his improbable dream, only to find fans, fortune and fame.  Four years ago, when Cameron Silver opened the Los Angeles vintage-clothing shop Decades, his block of Melrose Avenue was hardly a fashion mecca.  "There was nothing but a few furniture stores, a falafel stand and FOR LEASE signs," says Silver.  That rundown look was soon replaced by trendy neighbors like Miu Miu and Daryl K.  And now, Silver says with some amazement, New York is taking its cues form him.  Last year he opened a Decades boutique in Barneys on Madison Avenue, and put vintage - a popular L.A. street look - on the backs of Upper East Side societies.  Says Silver: "I can't believe I have these rich New York ladies dieting to wear clothes they gave away 30 years ago."