I have yet to see anything in GF or Pt II that indicates Vito did not want Santino and Fredo to follow in his footsteps. I've already listed plenty of evidence he had "legitimate plans" for Michael (since he was a baby). What makes you believe Vito did not want (at least) Santino to follow in his footsteps and lifesyle? He always asked him (Sonny) to be present when he conducted business.