J.D. Vance can't think of any better Christian theologian than Diddy ... just kidding.

Take a look at the video ... the veep visited the Richard Nixon Library Thursday, when he was asked about a chapter in his memoir titled "More Money, More Problems" -- a reference to Diddy's song with The Notorious B.I.G. -- and he felt compelled to poke fun at the now-incarcerated music mogul by calling him a "great Christian theologian."

Obviously, the President's right-hand man said it in jest, and drew some big laughs -- particularly from himself.

He quickly added ... Diddy's been the total opposite of a model Christian in recent years. So, yeah ... J.D's not living under a rock, no matter what his haters say.

If you're not tickled by his Diddy dig ... VP Vance also had a wild take on Nixon's legacy.

J.D. quipped if Watergate happened tomorrow, it would only be a headline for 12 hours, and added ... "the idea that it would've taken down a presidency is crazy."

Here's the thing, he was NOT saying that part in jest. Watch the clip ... he really believes Nixon got a raw deal. 🤔