We spent 5 nights at the campsite at Larochette in Luxembourg 4 years ago and visited Vianden while we were there. Vianden is pretty but the streets are very narrow and steep so parking and driving are an experience - getting to the castle involves a one way system round the hills. We didn't go to the castle but the rest of Vianden took us part of an afternoon to see (including a coffee stop where we got the most amazing cakes).
Luxembourg is such a small country that if you run out of things to see and do in Vianden there are plenty of other little towns to visit. Luxembourg city is beautiful but a nightmare to negotiate by car. Both times we drove into it we got hopelessly lost, as did my in-laws.

thanks elaine ,
we are going by coach , so will avoid all of the hazzards involved .
good job by the sounds of it .
bill