With Vito's connections I'm sure he could have set Fredo up in a legit business or two, union,trucking company,etc. Fredo wouldn't need much in the way of brains,just show up and get a check, and steer some business to the Family now and then. I think that Vito's (and to lesser degree Sonny's) biggest sin was in not grooming Fredo for a better position, Sonny could have mentored his younger brother, and even Tom could have helped Fredo in gaining a more lucrative and less volatile life.