How can I take a truly vintage piece and make it look trendy versus old/grungy?

How can I take a truly vintage piece and make it look trendy versus old/grungy? LR: Don't wear a vintage look head to toe. Maximum 2-3 retro pieces per outfit, and mix them in with your modern skinny jeans or sandals. You don't want to actually BE sandy and Danny from Grease, just inspired by them!
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