Reaching Vahlen's office | Mission 5 - Siwa Oasis Sniper Elite III Guide

Vahlen's office is located in the South-Western part of the city (point 13 on the map) and the best way to approach is from the South (the above screenshot). Of course, you need to keep in mind that there may be, at least, several enemies around but, with appropriate precautions, all of the enemies can be eliminated with cunning attacks by surprise (a sniper rifle, a suppressed weapon, melee attacks).

The last-but-one War Diary

Take the stairs but, BEFORE you enter the office, check out the neighboring building on the left. Here, you can find the last-but-one War Diary - Axis Account: Missing in Action. After you obtain the collectible, head towards the current destination point and see the cutscene.