I agree that it was a wrong decision. Puso writes quite openly in the book that Hagen felt so. He thinks that Vito will refuse, "but what was worse, Hagen was convinced that for one of the few times in his experience, the Don had not thought things through. He was not looking far enough ahead." That's a mistake indeed!
But I believe Vito made one more mistake.
Book says that Connie "displeased her father with her choice af a husband. If he saw clearly enough what kind of a person Carlo was, he had to prevent it. It was easy for Vito to make them split, or separate them, or invent something to send Carlo away, considering what a fool, coward and money-loving villain he was. He had to meddle.