Hmm. My VS450 has two 6 pin connectors, must be a different version to yours...
The 6850 doesn't use a huge amount of power. If you have two 4-pin connectors free you could always buy a molex to pcie converter and use that for the second slot?
The way I see it, that card has 2 connectors for a reason. It probably won't work with just one. There are adapters to convert a couple 4-pin Molex connectors to a 6-pin PCI Express that you can use.