You can output NTSC mpeg @ 23.976fps with 2:3 pulldown in vegas. You don't have to use 29fps for your output. when you select your output chose the profile "DVD NTSC" and then click on "custom" then in the window that pops up click on the "video" tab. Change your aspect ratio settings and change the framerate to " 23.976 + 2:3 pulldown" in the framerate pulldown menu on that page. You will notice that the field order box will now be greyed out. Now change the bitrate settings if needed and save the new profile.

I assume your Vegas project is set to 23.976 fps and not 29.970fps? Did you perform a pulldown removal prior to importing the video into vegas or are you working from a direct mpeg2 dvd rip?

I have the NTSC version of AOTC and it is 23.976fps with a 2:3 pulldown. An NTSC DVD will usually report the framerate as 29.970fps because the pulldown has been applied so you need to process the video first to remove the 2:3 pulldown or you will duplicate frame issues when rendering an output