I posted above it was Arthur C. Clarke. I actually did some research and found that reference with a Google search. But Van Vogt kicked off a memory -- it sounded right -- so I searched a little more. I found the bet is said to have been with Joe Haldeman or Heinlein or John W. Campbell Jr. or??? Apparently no one can really pin it down. But now I recall reading years ago it was with Van Vogt. Remember "Slan?"
For what it's worth, I had always heard it was Campbell.
I recall the "bet" being published by Campbell in an editorial slamming Hubbard for his founding of a "fraudulent" religion and it was there that Van Vogt was cited as one of those in the discussion. It may have been a "round table" discussion in which Hubbard made the claim that he COULD create a religion.
I loved "Slan", "The Weapon Shops of Isher" and "The worlds of Null-A" (null A meaning Non-aristotelean thinking) series. One little noticed gem is "The Voyage of the Space Beagle." Except for the Weapon Shop series, Van Vogt'w works seemed to show that he was convinced that mankind was "perfectable" and his works tended toward that idea. The Weapon Shop series is a must read for anyone who thinks the 2nd Amendment IS important.