Microsoft Visual Source Safe is a proprietary version control system. Free software alternatives are listed at VersionControl.
I’ve used this before, and it is very useful, but stopped because file access became unbearably slow. --PatrickAnderson
Also note the package seems to be no longer maintained. 
See also http://ftp.gnu.org/old-gnu/emacs/windows/docs/ntemacs/contrib/source-safe.el --Andy Chambers
Documentation is scarce. I did this to get something working:
One problem I’ve found is with files that are not version controlled within a dir that has version control enabled otherwise.
;; example setup (require 'vc-vss) (setq ;; not sure what to set vc-vss-path to... ; vc-vss-path "//sourcesafe/someDatabase/users/you/ss.ini" ; vc-vss-path "c:/Program Files/Microsoft Visual Studio/VSS/srcsafe.ini" ; vc-vss-user "username,password" vc-default-back-end 'VSS ; diff-switches "-DV" vc-vss-open-async t vc-vss-open-async-rw t) (add-to-list 'exec-path "c:/Program Files/Microsoft Visual Studio/VSS/Win32") (add-to-list 'vc-handled-backends 'VSS) ;; optional: these may be needed if you are not "logging into the domain" (setenv "SSDIR" "\\\\sourcesafe\\someDatabase") ;directory of the ss.ini for that database (getenv "PATH") (setenv "SSUSER" "username") ;without domain (setenv "SSPWD" "password")