Well, there is the issue of the "Broadcast Flag" that Vista turned on and that's just for starting the discussion. Try this. Disable the Broadcast Flag in Vista. (google if you don't know how.)

The low signal is a sign the hardware has developed a fault. We can't tell from here but given Vista hasn't been offered in years you would have had it fixed if it didn't work before. So always tell the full story.

And because replacing the WiFi card in laptops is too expensive, we'll use other replacements.