Well, a Consigilere is only an advisor. The Don is the principle and should know more, be more aware that anyone else in the family. Vito was worried enough about Sollozzo to send Luca into harm's way. However, he was not worried enough to protect himself. If Vito was not so worried about physical protection, I don't blame Tom for not worrying that much either.

I also caution about assuming that Vito's concept of legitimate was similar to anyone else's. Vito's plan for Michael's legitimacy would have been to assist with illegitimacy.