The vintage year on an eiswein label refers to the year when the frozen grapes were picked.
Ok, this was a tricky one. In general, the vintage listed on any wine bottle refers to the year when the grapes were harvested. But eiswein (or icewine in English), made from grapes frozen naturally on the vine, is an exception. Grapes for eiswein are sometimes harvested in January of the next year. In this case, the vintage on the bottle would be the previous year, the primary year in which the grapes were grown.