Nintendo doesn't even sell/support that wifi adapter anymore. For the price difference between that and a wireless router, it's not worth the hassle. You should just sell it and rid yourself of the headache.
I seem to remember the Nintendo wifi adapter having issues with Vista.
And your general experience with Vista is the norm: it's a bloated piece of slow junk. On any given hardware, it will run 25-33% slower than XP on the same hardware. There's really nothing of value to the consumer in Vista... Microsoft mainly just produced it to generate more income for themselves and to pacify the media companies who want more DRM. And the sweet thing is, they can make people upgrade simply by ceasing support for XP.