Get a bit rate calculator (I believe you can find one for free on http://www.vasst.com or if you have UltimateS, you can use the one built in that one) and see what bit rate you need to set your file to.
If you are going straight to DVD and never have intentions on using the HD footage on BluRay or the such, set up your timeline properties to match a standard definition project. If you do want to utilize the HD file at some point, just open up the file in vegas and let it set the properties for you. When rendering out for SD, choose DVD Architect Wide Screen and tweak the template according to your bitrate calculator and render the audio out as AC3 seperately. That should do you.
j razz