According to Blauw Bloed
the scions of Otto (Willem-Alexander) and Walram (Henri) used the exhibition at Burg Vianden
for a flash-course "History of the House Nassau" to their interested spouses.
The fact that the Orange-Nassaus got the Grand-Duchy of Luxembourg after the Congress of Vienna in 1815 was mainly based on their widespread territories as Counts of Vianden.
The Orange-Nassaus were Counts of Vianden. But when King/Grand-Duke Willem III died in 1890, not his daughter Princess Wilhelmina but the head of the other Nassau branch became Grand-Duke of Luxembourg and Count of Vianden.
It seems that it is agreed that bot the Grand-Duke as well the King keep using the title Count of Vianden in the so-called intitulé
(the Etc. Etc. Etc. in the titulature).