Looks like LH has closed one of the last loops to use USD vouchers for flights that do not originate from the US (Or for that latter any vouchers from a country without TSC). When trying to book DUS-ZRH on LH USA, the system displays this:
Based on your request we have routed you to our ticket shop in Germany. The displayed currency is Euro (EUR) and your payment card will be charged from a merchant in Germany.
The price is given in EUR and LH vouchers that were valid for US are no longer accepted. It was good as long as it lasted...