Michelle Stafford, who plays slinky, kinky Phyllis on The Young and the Restless, continually pushes the envelope, unlike many daytimers. Introduced last fall, Phyllis with a penchant for attempting murder and an endless supply of lingerie has already entered soap's bad-girl hall of fame. ''Phyllis is so evil,'' says Stafford, ''I figured I'd probably be out of work in 13 weeks, so why not go over the top?'' She went way over this month when she donned a black vinyl teddy. ''Wardrobe suggested it and I said, 'Far out!''' says Stafford, who helped shop for the garment. ''It should have come with a whip.'' Stafford, a Chicago native and ex-runway model, reports that fans ''scream at me with such venom,'' in appreciation of her va-va-va-voom villainy. ''I was the same way when I watched soaps,'' she says. ''I hated the nice people.''