Is there a strategy to improve chances of encountering Vaal areas?

Question

I am trying to do the "no stone unturned" achievement and, as the image shows, I am 1 stone short.

Having done the labyrinth multiple times, I am quite sure that I am missing only 1 "Vaal letter", found at the end of Vaal side areas.

As the letter found is random among the pool of possible ones, and since it has been a while since I started actively looking for them, I am wondering if there is a good strategy or a proven series of areas that are fast to check in order to hasten the process a bit.

Answer 1

You can actually force a Vaal area to spawn.

All you need is either the map device (earned when you get Zana to level 2), or an access to the Eternal Laboratory (Merciless difficulty, act 3, it's the area after the Solaris Temple level 2). Plus an Atziri fragment.

Drop a single Sacrifice Fragment (or Mortal Fragment, though it isn't recommended) in it, activate, and it will open a random Vaal area.

It's also a great way to force your chance into getting Midnight fragments to drop!