I will repeat what I have said elsewhere. Michael and Santino both resented and were a bit jealous of Tom's relationship with Vito. In many ways Vito understood that Sonny was nothing without Tom, and that if Michael was to have a "legitimate" careed, only Tom could be the intermediary.
The result was all the mean spirited things both Sonny and Michael constantly said to Tom.
Never thought of it like this before. Why do you think that they resented him at times? I think michael and sonny both loved Tom. But there could have been some jealousy there. I’m sure Tom did refer to Vito as his dad but in the scenes we saw he didn’t.
