The main ones really been the treatment of Olise and Eze. From what's been said from people that would know what's going on, Patrick Vieira had a tough love approach with both of them, and Eze in particular looked destroyed of all belief and confidence as he played in a deep midfield role in Patrick's last game against Brighton. I think that's a big thing that caused Parish and Co to lose faith in the project.
Weirdly, the tough love he had with Eze and Olise was the opposite of how others were treated. The Athletic reported Shaun Derry was released (although Ambrose reckons hes always been on the pay role) because he ripped in to the senior players for there performance against Everton. Patrick didn't agree with it and removed him from the backroom staff. I imagine part of that was Vieira felt guilty for trying to push a 4-1-4-1 formation on the team which he changed after that match and Derry was the fall guy for a 'clean slate'.
The one gripe I had with Vieira was his second half substitutions which was a situation that didn't improve over time. That said, from what Osian Roberts full job title was, he may not have been well advised by the experienced head next to him.
I still think its harsh we got rid of Vieira before the fixture turn but the results Roy got with Eze as the focal point justifies there decision. I'm glad he's gone to France, as if he went to a team like Forest in November and they went above us, the atmosphere would be very toxic.