This one will set the undersample rate to .75. Adjust the script as needed.
/**
* This script will set the UnderSampleRate for all selected events.
*
* Written By: Edward Troxel
* Copyright 2005 - JETDV Scripts
* Modified: 01-10-2005
**/
//Go through the list of Tracks
var trackEnum = new Enumerator(Vegas.Project.Tracks);
while (!trackEnum.atEnd()) {
var track : Track = Track(trackEnum.item());
//Go through the list of Events
var eventEnum = new Enumerator(track.Events);
while (!eventEnum.atEnd()) {
var evnt : TrackEvent = TrackEvent(eventEnum.item());
if (evnt.Selected) {
var ve : VideoEvent = VideoEvent(evnt);
ve.UnderSampleRate = .75;