TarkovHead

In this quest you have to obtain the disk with unknown data through a unique sacrifice at the Cultist Circle in the Hideout.

The items that need to be sacrificed are: Programmer's diary (Copy), Expeditionary fuel tank (any durability) and Zibbo lighter.

Only these 3 items must be put into the Cultist Circle for the sacrifice to work.

The first word in the phrase "Diary. wvyglv. oiphxifq: sbb onfz qulve. rgx: ssemddvyl" mentioned in the quest's description is hinting at where to get the key. The first letters of words in the table of contents in Programmer's diary (and likewise its copy) form together the word "UNHEARD". Using this as a key for the Vigenère cipher returns "circle. location: you know where. add: flammable", hinting at the sacrifice to obtain the Disk with unknown data.