Yes, the Vault API and DLLs change between versions, so code compiled against AutoCAD Vault 2012 libraries will not work reliably with Vault 2015 or newer. You’ll need to reference and build against the matching Vault SDK for each version. There’s no true “write once, run everywhere” approach with Vault, but you can structure your code to use abstraction/interfaces and compile separate builds per version, or use conditional references to target multiple Vault releases.