I'm currently involved in a technology upgrade project in my company, and would like to know if VSS6.0 is compatible with latest Visual Studio 2015?
VSS is not included since VS 2010 due to performance issues. According to
Can I Use Visual Source Safe with Visual studio 2013?
Visual Studio 2015 Professional and Visual SourceSafe (VSS)
you can choose "Microsoft Visual SourceSafe" plugin from Tools -> Options -> Source Control.
I suggest you to use TFS/CVS/Git/SVN rather than VSS for better version control of your application.