The dungeon, Ferdinand used to have the territory and should have sent surveyors who probably found it and they made a massive deal about finding one bringing prosperity and riches.
So if Ferdinand found a dungeon and didn't tell his king so as to monopolize it but soon after lost it to Vans family and they also found it then word will reach the king who might be less than happy that a Viscount failed to mention such an important resource being found in their territory, clearly showing that they are unfit for nobility. (or their heads.)
Option 1 to cover up his folly: Sending his last unmarried daughter to marry the current lord, bringing them into the family and returning the territory to him, or at least giving him an excuse to generously survey the land for the current lord and just so happen to find the dungeon for the first time.
Option 2: As mentioned by Panamera there may or may not be an army of knights at the ready to invade and take the territory by force, but that is likely to be a last resort since it would bring a lot of attention and scrutiny.