I remember reading somewhere that Spengler and Stantz, the latter to a lesser degree, “helped” Venkman a lot during the college days.
It's my hypothesis that Venkman, being the sly devil he was, tagged along with Spengler and Stantz into psychology and parapsychology because he knew he could coast into a doctorate with Egon and Ray doing most of his work.
But, just because Venkman may not have done all of his own work, doesn't mean he didn't grasp the concepts; I always imagined he was too busy perfecting “The Venkman Charm” with some co-ed to worry himself too much with school.
—Ecto1A