Vito is wise enough to realize that nothing lasts forever. Don't forget, at the time of the Sollozzo meeting, Michael is not in the picture. Vito can't count on him as a successor. Vito has to seriously consider how his involvement in drugs might erode his political and legal support. As the novel points out, that support has been the cornerstone of Vito's power since day one. The Tattagias were involved in the "thin" drug traffic at the time. Vito's involvement would have greatly expanded it and made it very high profile. He probably agreed with Tom that drugs were the coming thing. But he had to deal with things as they were now and within a shorter window of time. He had to balance the consequences of getting involved i nthe drug traffic against teh loss of his political support which might very well have undemined his other endeavors.

As far as there not being enough time, I think that Vito looked upon Michael's involvement in politics as a way to enhance his nefarious undertakings. I don't think legitimacy was in either Michael's or Vito's mind.