I think Michael himself didn't want a son like Vincent.He would do anything for turning legitimate,following his father's wish:
"Senator Corleone...".Also states this in #3.
Kay never believed him,never realised his hard efforts to stabilize stuff.Michael's mistake, as Fredo's,was to marry an American woman.Would Apollonia ever kill his child?
Also one more comment:

While Michael was away for buisiness,Kay couldn't leave the house,right?I bet that even her doctor was made to go in Lake Taho to examine her.How did she arranged the abortion?Another hint perhaps for Tom?