You can use a script written by one of the people at SOFO. It will create both the MPEG2 and the AC3 files in one operation. I think this script is on the Vegas CD, but here it is again, in case you can't find it. Just save this in a folder with a file name that has the extension ".js" (e.g., "DVD Architects.js"). Select "Scripting" then "Run Script" from the Tools menu and select this script to run it. You can also assign this script to one of the shortcut keys so you can start it just by a single shortcut key combination.
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/**
* The following script renders a project in two stages for use in DVD
* Architect. The first stage renders the video as MPEG-2 (NTSC),
* second renders the audio as either AC-3 or 48KHz Wave.
*
* Revision Date: Feb. 10, 2003
**/

import System;
import System.IO;
import System.Windows.Forms;
import SonicFoundry.Vegas;

var videoRendererRegexp = /MPEG-2/;
var videoTemplateRegexp = /DVD Architect NTSC video/;
var audioRendererRegexp;
var stereoTemplateRegexp = /Stereo/;
var surroundTemplateRegexp = /Surround/;
var fourtyEightKHzRegexp = /48/;
var audioTemplateRegexp;

try {

var renderStart = new Timecode();
var renderLength = Vegas.Project.Length;
if (Vegas.SelectionLength > new Timecode()) {
var msgBoxResult = MessageBox.Show("Render selected region only?", "Region Selected", MessageBoxButtons.YesNo);
if (msgBoxResult == DialogResult.Yes) {
renderStart = Vegas.SelectionStart;
renderLength = Vegas.SelectionLength;
}
}

var projDir, projName, videoOutputFile, audioOutputFile;

var projFile = Vegas.Project.FilePath;
if ((null == projFile) || (0 == projFile.length)) {
projDir = "E:\\My Video\\";
projName = "JHM";
} else {
projDir = Path.GetDirectoryName(projFile) + Path.DirectorySeparatorChar;
projName = Path.GetFileNameWithoutExtension(projFile);
}

videoOutputFile = projDir + projName + ".mpg";
videoOutputFile = ShowSaveFileDialog("MPEG-2 Files (*.mpg)|*.mpg", "Render Video File", videoOutputFile);
if (null == videoOutputFile)
throw "operation canceled";

var mpegRenderer = findRenderer(videoRendererRegexp);
if (null == mpegRenderer)
throw "failed to find MPEG-2 renderer";

var videoTemplate = findTemplate(mpegRenderer, videoTemplateRegexp);
if (null == videoTemplate)
throw "failed to find video template";

audioOutputFile = Path.GetDirectoryName(videoOutputFile) + Path.DirectorySeparatorChar + projName;
var audioFileFilter = "AC-3 Files (*.ac3)|*.ac3|Wave Files (*.wav)|*.wav"
audioOutputFile = ShowSaveFileDialog(audioFileFilter, "Render Audio File", audioOutputFile);
if (null == audioOutputFile)
throw "operation canceled";

var audioExtension = Path.GetExtension(audioOutputFile);
if (audioExtension == ".ac3") {
audioRendererRegexp = /AC-3/;
} else if (audioExtension == ".wav") {
audioRendererRegexp = /Wave.*Microsoft/;
} else {
throw "unknown file extension: " + audioOutputFile;
}

var audioRenderer = findRenderer(audioRendererRegexp);
if (null == audioRenderer)
throw "failed to find audio renderer";

if (audioExtension == ".wav") {
// just do 48KHz stereo.
audioTemplateRegexp = fourtyEightKHzRegexp;
} else {
// if the project is surround, use the surround template,
// otherwise use the stereo template.
var audioProperties : SonicFoundry.Vegas.ProjectAudioProperties = SonicFoundry.Vegas.ProjectAudioProperties(Vegas.Project.Audio);
if (audioProperties.MasterBusMode == AudioBusMode.Surround)
audioTemplateRegexp = surroundTemplateRegexp;
else
audioTemplateRegexp = stereoTemplateRegexp;
}

var audioTemplate = findTemplate(audioRenderer, audioTemplateRegexp);
if (null == audioTemplate)
throw "failed to find audio template";

var renderStatus = Vegas.Render(videoOutputFile, videoTemplate, renderStart, renderLength);
if (renderStatus != RenderStatus.Complete)
throw "render not complete";

var renderStatus = Vegas.Render(audioOutputFile, audioTemplate, renderStart, renderLength);
if (renderStatus != RenderStatus.Complete)
throw "render not complete";

} catch (e) {
MessageBox.Show(e);
}

// find a renderer that matches the given regular expression.
function findRenderer(re) {
var rendererEnum = new Enumerator(Vegas.Renderers);
while (!rendererEnum.atEnd()) {
var renderer = rendererEnum.item();
if (null != renderer.FileTypeName.match(re)) {
return renderer;
}
rendererEnum.moveNext();
}
return null;
}

// find a render template that matches the given regular expression.
function findTemplate(renderer, re) {
var templateEnum = new Enumerator(renderer.Templates);
while (!templateEnum.atEnd()) {
var template = templateEnum.item();
if (null != template.Name.match(re)) {
return template;
}
templateEnum.moveNext();
}
return null;
}

// an example filter: "PNG File (*.png)|*.png|JPEG File (*.jpg)|*.jpg"
function ShowSaveFileDialog(filter, title, defaultFilename) {
var saveFileDialog = new SaveFileDialog();
if (null == filter) {
filter = "All Files (*.*)|*.*";
}
saveFileDialog.Filter = filter;
saveFileDialog.Title = title;
saveFileDialog.CheckPathExists = true;
saveFileDialog.AddExtension = true;
if (null != defaultFilename) {
var initialDir = Path.GetDirectoryName(defaultFilename);
if (Directory.Exists(initialDir)) {
saveFileDialog.InitialDirectory = initialDir;
}
saveFileDialog.DefaultExt = Path.GetExtension(defaultFilename);
saveFileDialog.FileName = Path.GetFileName(defaultFilename);
}
if (System.Windows.Forms.DialogResult.OK == saveFileDialog.ShowDialog()) {
return Path.GetFullPath(saveFileDialog.FileName);
} else {
return null;
}
}