If Vito were still alive, would Fredo have been involved in the hit. Would there even have been a hit? It's an interesting topic. Fredo was sent to Vegas in '46 and Vito died in '55. That's nine years. Was he on Vito's "shit list" the whole time? From the novel:

Hagen shook his head. Freddie had greeted him much less affectionately than Michael, but Hagen understood. Freddie was on his father’s shit list and Freddie naturally blamed the Consigliere for not straightening things out. Hagen would gladly have done so, but he didn’t know why Freddie was in his father’s bad graces. The Don did not give voice to specific grievances. He just made his displeasure felt.

And then later:

Hagen saw that this had caught Michael by surprise. They looked at each other. This was perhaps the real reason the Don was displeased with Freddie. The Don was straitlaced about sex. He would consider such cavorting by his son Freddie, two girls at a time, as degeneracy. Allowing himself to be physically humiliated by a man like Moe Greene would decrease respect for the Corleone Family. That too would be part of the reason for being in his father’s bad books.

Why didn't Vito find a way to bring him back into the fold? Nine years is a long time. And Fredo's resentment would have festered the whole time. So, in a way, Vito's disapproval of Fredo's somewhat dissolute lifestyle was the source of all the family's (small f) future catastrophes.

What about the other main player:

Vito died in '55, and the Tahoe hit occurred in '58. Three years. I reckon - and these are purely my thoughts - that Vito would have been a calming influence on Michael. But with his death, Michael was unleashed and all his expansionist plans would come to the fore. Let's not forget that he was an alpha male in excelsis, and a bit of a control freak. There was absolutely no-one to slow him down. So Roth (c. 1946-55) might well have welcomed the Corleone's investments in Nevada, especially with Vito's steady hand counselling Michael. But Roth (c. 1955-58) was probably horrified at Michael's true expansion plans. And so he hatched his plan. I've made my thoughts on this plan known in other threads, but it happened. It needed an inside man, and ironically perhaps, Fredo being on the outer made him the obvious choice.

So, Vito's dying, Fredo's resentment and Michael's thirst for power made things sort of inevitable.