Regis Philbin -- Bails on Vespa During Letterman Taping

Regis Philbin tried to tame a mighty Vespa on the streets of NYC this afternoon ... and lost.

The 80-year-old was riding the wild scooter for a bit with David Letterman ... when Regis tipped his bike and spilled onto the asphalt.

Philbin grimaced as he hit the ground -- but being the tough son of a bitch that he is, Reeg got right back up and finished the bit.