Well, we all know DeNiro's a skilled enough actor that he probably could've made it work playing his own grandson. Only thing, Vincent and therefore Mary, would by then have to be middle aged, and as stated Michael would have to be even older than we see him. Might've worked...but FFC probably decided it wasn't worth the complexities involved for all.

In my opinion, the character of Vincent was unnecessary in the first place, and should never have been created.

Apple