While the Church has made no official pronouncements regarding his vision, history acknowledges that it was some inner experience that caused Constantine to change the course of history in granting freedom to Christianity. While due to some of his un-Christian actions history does not show him to be a saint, it does call him great. He remained a catechumen until the end of his life when finally he laid aside his royal purple, took on the white robe of the neophyte and received Baptism. He died shortly thereafter. So we trust that he is a saint in heaven, though not a canonized one.
I encourage you to read the full section on Constantine in the original Catholic Encyclopedia:[LIST]
*]Constantine the Great[/LIST]Fr. Vincent Serpa, O.P.