Based on the study posted below my guess is L-lysine is stable under proper storage conditions with very little change in composition. I can only guess about your vitamins. What I think is most of the products like vitamins have to be stable compounds to be on the market and degrade very slowly. In the absence of oxygen or heat probably no degradation would occur. In a few weeks I’ll ask my friend that is working for a pharmaceutical company and get a better answer.
Eruopean Food Safety; 01-2015
Scientific Opinion on the safety and efficacy of L-lysine monohydrochloride
produced by fermentation with Escherichia coli for all animal species based
on a dossier submitted by HELM AG on behalf of Meihua Holdings Group
2.5. Stability and homogeneity
2.5.1. Shelf life
The shelf life of L-lysine HCl was studied using three batches, packed in 25-kg paper bags and kept at 25C and 60% relative humidity (RH) for a two-year period. No losses of the active substance were observed after 24 months.