Not sure if you figured this out yet, but if not…
yes, you can do it the way the other user is suggesting. …then when you bring that rendered video clip back in, you will have to right click on it once it is on the timeline, select properties, then ‘Media’ tab, then under “Alpha Channel”, select “Straight”. This should do what you want, assuming it was rendered with alpha channel, as I believe you have done.
However, for future reference, there is an easier (or maybe I should say faster) way to do this on the timeline without having to prerender any clips.
Place white text on a track UNDER the video file that you want to show through the letters. Click the ‘Make Compositing Child’ button on the text track, then click the ‘layer effects’ button on the video track and select “Multiply (Mask)”.

Create a Mute envelope on the video track to mute the track when you no longer want to see the text for the rest of the video so you can see through to the rest of your video. If you want a dissolve, transition to a white solid color on the video track that is in “multiply mode”.
I’m using Vegas 12 and 13, but I’m pretty sure that this is also possible in 11.
Hope that makes sense and does what you’re trying to do. Good luck!