Vergil

Indeed it worked, thank you! Special encounter at council worked too, got the orders quest now.

Upd: Went to Fort Haledon, sucessfully finished the quest.

If anyone needs:

I've changed guard dialog in Scripts\t31Olgaidlg.txt like this:

begintalknode;
tag = 2;
state = -1;
nextstate = -1;
condition = 1;
text1 = "The vahnatai guards stands motionlessly at attention, alert for any threat that might invade and try to harm their leaders. When you try to speak with them, they ignore you.";
code =
set_spec_item(14,1);
break;

Just speak to Olgai guard and it will give you letters.

Edit: In these moments, I wish item editor was still present in the game.

Edit2: I've sucessfully finished the game, thank you Slartibus, you're saved my game progress, time, and nervs:)

Vergil, can I ask if you also had visited Fort Haledon before you got to looking for the evidence letters?

I haven't yet, I'm aware that I can't take down the fort without orders, and I can't get them without these letters.

Edit: Is it possible, that I couldn't obtain letters, because my special item list is full?

I'm having exact same problem. Finished 2\3 main quests, this is the last crystal soul, and it seems I can't get letters from the book, or chest or shelf or anything. Double checked every corner and chest, everything. Checked special items - no vahnatai letters. I've finished quest slaying quest, went back, turned it in. Went to council, still does nothing. Any help? It seems like a bug.