Letterman brought mistress along… on family vacation.

I got nothing, sorry.

And the No. 1 reason David Letterman is a cad: The “Late Show” host brought the assistant he was sleeping with on Caribbean vacations with his wife and their young son, sources told The Post.

Letterman’s wife, Regina Lasko, had no clue the comic was having an affair with his pretty, much younger assistant Stephanie Birkitt — a $200,000-a-year employee — when she was tagging along on the cozy, romantic trips, sources said.

Except… wait. The extortion attempt was apparently being scheduled around Cronkite’s memorial service. Now there’s some symbolism for you.

Via JammieWearingFool, via Hot Air Headlines.