I agree that Vito was generous in his treatment of Tom. He not only raised him as a son, but the novel says he offered to stake Tom to whatever career he wanted, no strings attached, no quid pro quo. Tom asked to work for Vito. And, Tom (presumably for reasons relating to the dissoluteness of his own parents), was almost pathologically dependent on the Corleones to be his family. I think the novel said he had nightmares, and he cried when Solozzo told him that Vito was dead. So, in many ways, Tom chose captivity.

But, the arrangements Vito, and later Michael, made for Tom assured he'd never stray off the range or get any big ideas on his own. It was a perfect arrangement for them, and for Tom under Vito. Not under Michael.