As it has been said, Egon and Ray helped Venkman through his post-grad work. There is never any direct reference as to how much help, though it is possible that computer hacking was involved.
Columbia University is one of the best schools, and certainly the most competitive. Doctoral candidates are thoroughly grilled so my theory is that Venkman did a lot of favors, made some powerful friends, and strolled through the dissertation with a brilliant oral presentation, backed up by notes written by E and R.
I'm sure there were those, like Dean Yaeger, who were critical of Venkman from the beginning and took great pleasure at breaking him and his department down. Venkman isn't very good at diplomacy, it seems.
You're right, he is a SMART GUY. He's street smart. He knows people. He can get what he wants and can persuade people to see things his way. He's a booster, a PR man…a con artist in many ways. He does see science as some kind of dodge or hussle to keep out of the private sector or working for a living.