The internet is littered with well written articles on the flaws of VSS. I would collect this as a body of evidence for moving away from VSS. Find a key requirement that VSS can't support (remote working, support on other OSs, tools integration) and use it to drive your issue. You then need to find a source control system that is a good match for your organisation's requirements - are you sure Subversion is that system? Set up a demonstration of your chosen system, and use this to prove its worth.
I implemented this change at a previous employer (first to CVS, and then to SVN), and while it was successful we had to build a lot of bits around the edge and rely on a lot of (sometimes unreliable) open source projects to get all the tools we needed. With hindsight I should have considered trying to evaluate professional tools such as Perforce, Vault or even Team System. Having evaluated these, I could have made a proper value judgement on whether CVS/SVN were worth their "free" price tag.
