It happens that VSLangProj.dll references EnvDTE.dll version 7.0.3300.0 while Visual Studio 2005 add-ins use EnvDTE.dll version 8.0.0.0, so this causes problems verifying assemblies with peverify.exe. See here how to solve this problem:
PRB: PEVerify.exe causes problem with EnvDTE.dll verifying Visual Studio 2005 add-ins
http://www.mztools.com/Articles/2007/MZ2007032.aspx