I have a file where the lady speaks with a prominent "s" and would like to reduce it. what is the best way to do that in Vegas Video?

Sibilance often occurs around the 6 to 8 khz band, if you have a good equalizer function in your audio program, or can add a vst plugin (there are numerous free graphic eq's, google is your friend) , try looping the sound and filtering in that band until it sounds better without deadening the speech...

To elaborate... A de-esser is what most audio engineers reach for to control excessive sibilance. The upper band(s) of a multi-band compressor can be used as a de-esser, if you have one. If not, search for one as Battle suggested.

FWIW, Although it's 'cheap and easy', the digitalfishphones "Spitfish" de-esser VST plug-in works as good as, and/or better than many $____ de-essers. (IMO) Cheap and Easy= free and only a few parameter adjustments.
digitalfishphones.com - free audio effects plugins (http://www.digitalfishphones.com/main.php?item=2&subItem=5)

If you are using Vegas Pro thre is something that can be helpful to get you started in the menu item..."apply non-realtime effects" right click on the audio clip, go to the above menu and find the Multi Band Dynamics FX. It has preset for reducing plosives and silibants. While it's not 100% it's a good place to start and can be adjusted.
Not sure if it's in the non pro version of Vegas if that's what you're using.
Of course it is something that can be done directly in Vegas.

+1 for Spitfish.

Yes just look for any plugin canned, obviously, "de-esser". It allows you to focus in on just he "ess" sound and lower it the amount you want. At some point it starts to be too much ans effects the rest of the track. I have the Waves Renaissance package that has a wonderful de-esser, but it also has a lot of other plug-ins you may not want to buy. Perhaps check iZotope to see what they have. I believe they have a "Nectar" plug-in that is all about the voice, and does many things to enhance a vocal track.