How to add referenced DLLs to VSIX?

Hi,

I have an Extension for Visual Studio which compiles into a VSIX container. There are plenty of referenced DLL for which I've set the "copy local" property to true. I want that those dlls are also part of the VSIX, but they are not. Interestingly, I never had this issue until I switched to VS2015. Have there been any changes regarding how/which dlls are added to VSIX? How can I get my "copy local" dlls to be part of the VSIX container?

Thanks & Regards,
Max