Get them to google for 'vss problem', 'source safe corruption' or simply look at the Wiki page for it. That ought to convince them that it's probably not a long-term viable thing for you to be betting such a vital part of your business on.
How big is your team? (ie, I mean how many members, not whether or not you're salad dodgers) Once you start to get more than half a dozen quite active users, VSS is going to give you headaches.
I seriously doubt that Microsoft use it (in fact, don't they use a customised Subversion or CVS variant?) and you've got to ask yourself - if the company don't eat their own dogfood, why would you eat it?