Hi,
I think the problem doesn't come from the Toolkit, but from your package that can't find the Xceed.Wpf.Toolkit.dll. When the new Visual Studio is started, it has to reference the Toolkit in order to have access to its controls. We are not working with VS Packages.
This may be a start :
http://stackoverflow.com/questions/13674782/vsix-cannot-load-file-or-assembly-of-a-referenced-dll
http://stackoverflow.com/questions/20001191/vsix-extension-uses-3rd-party-dlls-unable-to-load-one-of-the-dependency
I think the problem doesn't come from the Toolkit, but from your package that can't find the Xceed.Wpf.Toolkit.dll. When the new Visual Studio is started, it has to reference the Toolkit in order to have access to its controls. We are not working with VS Packages.
This may be a start :
http://stackoverflow.com/questions/13674782/vsix-cannot-load-file-or-assembly-of-a-referenced-dll
http://stackoverflow.com/questions/20001191/vsix-extension-uses-3rd-party-dlls-unable-to-load-one-of-the-dependency