It depends on what you have and how you want it in the end.
Subtitles in DVD is one thing, hard subtitles in video is another.

If you want to do DVD with subtitles it might be better to add them in DVD Architect if you have prepared text already. You can use some subtitle editor, they are more appropriate for that purpose than Vegas.

If you only need to add small amount of hard subtitles, you can use text generator on upper video track, manually create every text generator event, manually measure length and position, fill in text.

For a large amount of hard subtitles you can use subtitle editor to create .srt file, install ffdshow tryouts, play srt file as video and either capture this video or use video proxy (for example, make avi executable creates proxy .avi file that will play .srt file as video). Then you add proxy file to your project, set up transparency (if needed) and render in vegas.