During the conversation with Vargas, you can raise the topic of Ace's journal. It remains a mystery what actually befell the Ranger and who is responsible for his death so, you can take up to resolve this mystery. The first pages on Ace's journal can be found near the Radio Tower. One of them is on the ground, close to the tower itself,, in the middle of the area (use the function to highlight items, if you cannot find it). And the other one is on the corpse of the Ranger, in the cave to the East. You can return, with the pages, to Vargas, who will order you to the Rail Nomad Camp. It is not compulsory to collect the pages to pick up the trail. Also, you can talk about Ace to Matt Forrestal in the main hall of the Ag center or with Harold at the shop in Highpool. Thanks to them, you will get the clue to go to the Camp.
At the Rail Nomad Camp, visit the local bar, where you meet up with Samuel. It is him that has Ace's journal. If you have found the pages near the Radio Tower, or have talked to anyone about it, you can tell him that you know someone has been accompanying Ace. If not - you can discover the truth by using dialogue options. Once Samuel cracks and tells you what happened, he will give you the journal. You can either kill him or let him walk (Angela always kills Samuel, if she is on the party). Return the journal to Vargas at the Citadel. Then, take it back to Thrasher, at the desk near the museum, who will promise you to search through the old books for more information. Talk to him some time later. He will tell you that the trail leads to Los Angeles, which is rumored not to exist anymore... This is the end of this quest.
Concerns the quests:
- Find Ace's journal;
- Find the missing parts of Ace's journal.