Another inexpensive possibility is the NewBlue Scrubber FX, which include a NoiseReduction plug-in. I have not tried it, but you can download and test it yourself.
IMO, the NewBlue is the best bang for the buck. Pretty intelligent, and fast. It's not as good as the Sony or WAVES tool, but doesn't cost 300.00+ either.
Jeffrey Fisher has a DVD on the VASST site that teaches Noise Reduction in Vegas. Doesn't help with the $0 budget issue since either way you're spending some dough.
Although you get much better control with some outside NR plug-ins, I was shocked at how much you could do within Vegas alone. (I'm no audio guy but I play one on Television)