Mar 12, 2015 - 4:32 PM
The Interlaken Ost to Zweisimmen leg can be reserved, you can see so in the timetable. In practice though, reservations for those trains are not necessary, unless you travel in a large group. The same applies to Lucerne to Interlaken Ost. Perhaps Rail Europe does not have access to these particular reservations. If you want them, I would advice to book them at any staffed rail station in Switzerland as soon as you have arrived in the country.
The VIP seats are different: only a few trains have them and there are only 8 per train. These should be booked as soon as possible. As far as I know these seats are CHF 15. The lower amount of CHF 8 applies to regular seats. Rail Europe does currently offer a 20% discount on reservations. The seats can already be booked for April 20, but if Rail Europe can't do so, you can book with the local railway company MOB. The link is on our Golden Pass page, under "More on other websites". The front seats in the front carriage are the VIP seats you need to book.
By the way, if you also need a rail pass, the Rail Europe shipping fee will be waived as your order will probably reach the minimum amount of $399 for free shipping.
I hope this helps you out!