Okay, in my opinion, there are 2 points I'd like to make.
1) I can't see Anthony, the way he was portrayed in GF 3, ever getting involved in the business.
2) Vincent, growing up without any family but his mother, wanted too much to be a part of the Corleone family to ever turn on his cousin. Remember what he said to Mary in his social club about missing growing up with his cousins, etc...I think he was being sincere.