Others have posted that it does last forever in their soaps. Just checked the date on mine, and it's actually at 8.5 months with zero sign of browning in a soap with white, yellow, and orange. I am guessing that experiences vary depending on recipe, process, FO, etc.
My FO contained 1.8% vanillin, and my usage rate was 6% in a high lard recipe that was gelled on a heating pad. I did add the VCS to the oils, rather than mixing directly with the FO; that was also a suggestion made in one of the threads here.