Take a sad rag and make it better. That's what top-tier designers such as Betsey Johnson, Nicole Miller, and Jennifer Nicholson did when they altered thrift-store threads for the Vintage L.A. 2004 fashion show. ''Brilliant,'' declared emcee Minnie Driver, who admired Richard Tyler's reworking of a rummaged houndstooth dress. ''I just might take that home.'' The afternoon tea-party eventheld at the New Mart in downtown L.A.'s fashion district April 24 raised more than $100,000 for the Salvation Army Alegria, which provides housing for families affected by HIV and AIDS. So it wasn't just castoff clothes that found a better home.