The connections you get from the ÖBB schedule planner include a bus between Vaduz and Buchs SG - which isnt included in InterRail. So I'd recommend to use a connection with trains only - the train station for Vaduz is Schaan-Vaduz a little outside of the city. From there you have connections to Buchs SG where you can proceed to Luzern.
The reason why the ÖBB site also shows these connections by bus is because it has all regional bus services in Austria and Liechtenstein included to make travel planning easier. Otherwise they use the same database as the travel planner that is linked here at the forum.