Many trains from Basel to Interlaken.
Interlaken has two stations - West and Ost (=East), only 5 minutes apart. Know which one is closer to your hotel.
Interlaken is not on the main line that goes from Basel all the way through the alps down to Italy. Interlaken is on a side-spur of that line. Look at a map and you'll see what I mean - the fast trains for Italy head due south from Spiez.
Some trains will go all the way from Basel to Interlaken, many others will require you to make a switch in Spiez, only about 20 minutes before Interlaken. This is quick and easy, so don't let it worry you - you get off and the other train is waiting on the next track over. You'll use the same ticket for both legs.
It is all at www.sbb.ch/en
What is the address of the Viking landing point? I'm sure there is public transport, but if the Viking website doesn't say so, then this one might: www.bvb.ch/fahrplan-netz/online-fahrplan