Vito Andolini was obviously quieter and more cautious than his brother Paulo, who even as a kid of 12 (??) attempted to get revenge for his father's murder and himself was killed as a result.

Paulo, therefore, is a parallel to his nephew Sonny - hotheaded and impatient. Vito was more willing to bide his time and wait for his opportuntity for revenge (which he finally gets about 25 years later.)

But I think more than anything else Mama Andolini was just telling Ciccio that Vito was dumb in the hopes of letting him escape.