The VST 2.4 specification is discontinued and no longer available from Steinberg’s site. The specification and SDK are available from third-party sites, looking for vst 2.4 and vst 2.4 specification in search engines, though I fear they are distributed without permission. Would it be possible that Steinberg release the VST 2.4 specification under a free license (BSD, GPL…)?
The documentation for VST 2.3, including source code, is available from Yvan Grabit’s page at Steinberg.