The quest "House Telvanni: A Warning in Firewatch" is well designed and clearly intended to allow players who wish to decipher the message themselves to warn Vaneris Theneria immediately, without getting Nevrile Omayn's help first.  If a player does this, they can warn Vaneris and will then receive a single journal entry, TR_m1_wil_KtM_2a 101, instructing them to meet Vaneris at the Lost Crab and to inform Nevrile Omayn of what has happened.  But Vaneris will not appear at the Lost Crab until the player has begun the next quest, and Nevrile Omayn has no relevant dialog in this situation - the option "cryptic note" is no longer available, and the option "Vaneris Theneria" has not been added.  It is necessary to manually add the latter topic through the console to complete the quest.

The simplest fix would be for Vaneris' conversation option under "cryptic note", beginning "What? You have intercepted a coded message [...]", to add the topic "Vaneris Theneria" as well as the journal entry it already adds.  Alternatively, a new dialog item could be added to make it possible to ask Nevrile Omayn about "cryptic note" at this stage, which might be preferable as it will be higher up her dialog list and is the topic the player has previously been using to discuss this quest, so is what they are most likely to look for.