I would just use a nest project. Just drag the project of the person speaking and drop it on the track under the lower third and you're done. It will behave as a single piece of media and you can always right-click on it and open the original project if you need to edit any of the tracks. I work like this all the time. It's one of the best features in Vegas Pro IMHO.
~jr
http://www.johnrofrano.com
http://www.vasst.com