Yes, your issue is version compatibility. The Vault API changes between versions, so binaries built against Vault 2012 DLLs will not reliably work with Vault 2015 because of authentication and API differences. You’ll need to reference the matching Vault 2015 SDK assemblies and recompile your code. Unfortunately, there’s no single universal build that works across all Vault versions, but you can design your code to be version-flexible by using abstraction layers or conditional compilation so you only swap DLL references per version.