It is not clear if the Mind Flayer is controlling Vecna or vice versa. Nevertheless, the newfound insight promises a potential key to overcoming the ultimate villain in the storyline.
Warning: Major spoilers ahead for Stranger Things: The First Shadow
A Reddit user has shared an interesting scenario about how they can find Patty. In this version, when Joyce and Hopper will discover Henry Creel's responsibility for the events in Hawkins, they will link it to Patty Newby, Bob Newby's sister. Joyce might recall Bob mentioning that he possessed some of Patty's belongings, including the diary. The challenge will arise when they realize that Bob's house, where the diary might have been kept, was sold after his death.
The user also speculated that they could find the diary in Hopper's cabin. Hopper's dad was the chief and investigating the murders of the Creel family. In season 2, Hopper had many boxes in the cabin and one of them was labeled 'Dad'. There's a possibility that the diary might be stored in that box.
What are your views on this theory? Put down your thoughts in the comments below.