Finished playing 'Jedi: Survivor' today, and we all know that Tanalorr isn't safe from the Empire, but maybe not how they're going to find it.
The answer was given in 'Thrawn: Alliances'.
Sure, Tanalorr is hidden, and it's very difficult to navigate the Koboh Abyss, even with the necessary tools, but there is a people who excel at non-standard navigation.
Specifically, Force-sensitive Chiss.
Thrawn not only shares this secret with Vader, he teaches him how to navigate like the Chiss sky-walkers.
All that is necessary for this type of navigation is precognition via the Force, and touching the mind of someone who has been to the destination.
And he does it, successfully.
Vader has the knowledge and skill to reach Tanalorr, all he needs is someone who has already been there.
Just so happens that the Empire has parked a Star Destroyer right over Koboh after the end of the game.
I'm sure it's not related.