One of the best things is a graphic EQ, set for the maximum number of bands that Vegas Studio allows. Drop the slider at 60Hz all the way down. You may have to drop 120Hz down somewhat too to get rid of harmonics.
The noise reduction plugin will work wonders on this sort of noise, but it's $280 and not included with Vegas.
Looks like if you're using the 10 band EQ then the closest slider is 56. This will affect frequencies from about 40 to 80. With the 20 band EQ 56 will affect from about 48 to 67.
If you have the "Envelope" tab then you can doubleclick on the line at about 60 to add a node, and also slightly above and below so that you end up with 3 nodes. Drag the center one down. Repeat the same thing at 120Hz if necessary.