1) Buy a Bluetooth adapter (about $15), pry open the case, and set aside its electronic innards.
2) Open your vintage headphones—the larger the cans, the better—and remove the speakers and wiring.
3) Grab one male and one female ⅛-inch stereo jack, plus three short lengths of speaker wire: two split into a Y shape and a third split into an X. Solder one end of a Y-shaped wire to the right speaker's positive lead and the other two ends to the jacks' right channels. Repeat for the left speaker and the jacks' left channels. Solder the X-shaped wire to all ground terminals [see wiring diagram].
4) Plug the male jack into the Bluetooth adapter's female output. If necessary, drill holes in the headphones' cans to access the Bluetooth receiver's on/off switch and USB port. Otherwise, be sure the earpiece can be removed (to charge the receiver and turn it on and off).
5) Hot-glue the soldered connections to prevent electrical shorts, fit the receiver and wiring back into the headphones, and jam out.