of a hardware or driver incompatibility, or both. The problem is that I can't be sure, and neither can you, until and unless you try another wireless adapter - one that's guaranteed to work with Vista.
Network adapters are pretty simple devices, and usually they work well, but occasionally there's a driver issue with wireless adapters.
That's about all I can say, really. I know it's not much help, but I can't see an alternative - try another adapter, preferably a PCI one.