Remember that GF3 takes place entirely in 1979 and 1980. So Vincent becomes boss within two years of the beginning of the movie. And up until he meets Michael at the party, it appears that Vincent hasn't had that much direct contact with him. Moreover, Michael clearly wasn't even conceiving of making Vincent boss at the start of the movie. He seems content to retain Joey Zasa as head of the branch of the Corleone family remaining in New York. In GFI, by contrast, Michael kills Sollozzo in 1945, comes back a few years later, and then is under Vito's tutelage for another few years until Vito dies in 1954. So Michael clearly had a much longer "apprenticeship" than Vincent.