He's not Vandal's child, fortunately. Vandal never raised him. Now, he came across as an avuncular figure to Grant when they met, but he conveniently omitted certain details about how he became involved in Grant's life. The fact that he murdered Grant's biological mother, for starters.
I don't know whether or not Grant now knows the whole truth. I'm quite certain that he never found out during the course of his series.